Oxide Minerals 0939950030

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REVIEWS in MINERALOGY (Formerly:

"Short

Course

Notes")

Volume 3

OXIDE MINERALS DOUGLAS

RUMBLE,

III, Editor

The Authors: Ahmed El Goresy Max-Planck-Institut fUr Kernphysik Heidelberg, West Germany

Stephen E. Haggerty Department of Geology University of Massachusetts Amherst, Massachusetts 01003 J.

Stephen Huebner 959 National Center United States Geological Reston, Virginia 22092

Survey

Donald H. Lindsley Department of Earth and Space Sciences State University of New York Stony Brook, New York 11794

Douglas Rumble, III Geophysical Laboratory 2801 Upton St., N.W. Washington, D.C. 20008

Series Editor: Paul H. Ribbe Department of Geological Sciences Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061

MINERALOGICAL

SOCIETY

OF AMERICA

COPYRIGHT 1976 Reserved by the authors (Second printing,

1981)

PRINTED BY BookCrafters, Inc. Chelsea, Michigan 48118

REVIEWS IN MINERALOGY (Formerly:

SHORT COURSE NOTES)

ISSN 0275-0279 Volume

3:

OXIDE MINERALS

ISBN 0-939950-03-0

Additional copies of this volume as well as those listed below may be obtained at moderate cost from Mineralogical Society of America 2000 Florida Avenue, NW Washington, D.C. 20009 No.of Pages ~.

284

1

SULFIDE MINERALOGY, P.H. Ribbe, Editor (1974)

2

FELDSPAR MINERALOGY, P.H. Ribbe, Editor (1975; revised 1981) ~350 502 OXIDE MINERALS, Douglas Rumble III, Editor (1976)

3

4

MINERALOGY and GEOLOGY of NATURAL ZEOLITES, F.A. Mumpton, Editor (1977)

232

5

ORTHOSILICATES, P.H. Ribbe, Editor (1980)

381

6

MARINE MINERALS, R.G. Burns, Editor (1979)

380

7

PYROXENES, C.T. Prewitt, Editor (1980)

525

8

KINETICS of GEOCHEMICAL PROCESSES, A.C. Lasaga and R.J. Kirkpatrick, Editors (1981)

391

9A

AMPHIBOLES and Other Hydrous Pyribo1es - Mineralogy, D.R. Veblen, Editor (1981)

372

9B

AMPHIBOLES: Petrology and Experimental Phase Relations, D.R. Veblen and P.H. Ribbe, Editors (1981) (ii)

~375

FOREWORD Oxide Minerals was first printed in 1976 as Volume 3 of the Mineralogical Society of America's IN MINERALOGY"

"SHORT COURSE NOTES."

That series was renamed "REVIEWS

in 1980, and for that reason this, the second printing

Minerals, has been reissued under the new banner.

of Oxide

Only minor corrections

have

been made in this printing. Paul H. Ribbe

Series Edi tor Blacksburg, VA October 1981

EDITOR'S PREFACE and ACKNOWLEDGEMENTS The purpose of this volume is to provide, format, an up-to-date

review of the mineralogy

in a rapidly-printed, and petrology

inexpensive

of rock-forming

opaque oxide rr.inerals. It was the textbook for the short course on rock-forming oxide minerals

sponsored

by the Mineralogical

School of Mines, November be valuable

5-7, 1976.

not only to participants

Society of America

The contributors

at the Colorado

hope that the work will

in the short course, but also to others

desiring a modern review of the subject. The editor is grateful for invaluable

assistance

to Don Bloss, Paul Ribbe, and Southern Printing

in preparing

thanks are due Mrs. Margie Strickler entire text.

Elsevier

Scientific

the notes for publication.

for her outstanding

Publishing

course.

E. Leitz,

vital advice and assistance

Inc., C. Reichert

use in the ore microscDpy Finally, the Carnegie Director

the short

Corp.), Vickers

Instruments,

models of their microscopes

for

workshop.

I wish to acknowledge Institution

demonstration

Paul Ribbe, S.B. Romberger,

in organizing

(American Optical

Inc., and Carl Zeiss, Inc., provided

to reprint

Sciences Letters.

J.J. Finney, George Fisher, J.F. Hays, Jim Munoz, and E-An Zen contributed

Especial

work in typing the

Co. granted permission

figures from their journal Earth and PlanetaPy

Co.

the assistance

of Washington

of the Geophysical

provided

by the resources

of

through the good offices of H.S. Yoder,

Laboratory,

in organizing

the short course and pub-

lishing this volume. Douglas Rumble, III Washington, D.C. November 1976

(iii)

Jr.,

TABLE OF CONTENTS and EDITOR'S

FOREWORD USEFUL

PREFACE

(iii)

AND ACKNOWLEDGMENTS

REFERENCES

Chapter 1.

(ix)

The CRYSTAL EXEMPLIFIED

INTRODUCTION

CHEMISTRY and STRUCTURE by the Fe-Ti OXIDES

of OXIDE MINERALS

as

Donald H. Lindsley

.

L- 1

Techniques Magnetic properties

L- 1 L- 2

THE CUBIC OXIDE MINERALS

L- 4

Monoxides (Space group Fm3m) Spinel group • . . . .

L- S L- 7 L-12 L-IS L-IS L-18 L-22 L-24

Magnetite (Fe304) Ulvospinel (Fe2Ti04) Magnetite-ulvospinel solid solutions Maghemite (y-Fe203) . Magnetite-maghemite solid solutions Titanomaghemites . THE RHOMBOHEDRAL

OXIDES

L-3l

Hematite

Magnetic structure of hematite Curie, Neel, and Morin temperatures of hematite Ilmenite

.........•..

Crystal structure of ilmenite Magnetic structure of ilmenite

. .

Crystal and magnetic structure of hematite-ilmenite solid solutions . . . • . . • . . . . . . ORTHORHOMBIC

L-44

The structure of pseudobrookite (Fe2TiOS) The Fe2TiOs-FeTi20s series • . • . • . .

L-4S L-4S

OF RUTILE,

PSEUDOBROOKITE

L-4l

GROUP

STRUCTURES

OXIDES--THE

L-34 L-36 L-37 L-38 L-38 L-40

ANATASE,

AND BROOKITE

REFERENCES

Chapter 2.

L-S2 EXPERIMENTAL

INTRODUCTION Control

STUDIES

of OXIDE MINERALS

. . . . . of experimental

Oxygen fugacity Container problems

Donald H. Lindsley L-6l

conditions

. . . . . . . .

Experiments at very high pressures Minerals and phases considered FE-TI-O

L-47

or high temperatures

L-6l L-62 L-64 L-64 L-6S

SYSTEM

Fe-O join

L-66 L-67 L-67 L-67 L-68 L-69

. . . . • .

Wustite ..... Fe203 in magnetite Ti02 ..... FeO-Ti02 join . Fe203-Ti02 join (iv)

FeO-Fe203-Ti02(-Ti203)

join.

L-69 L-69 L-75 L-75

13000C isotherm Ti-Maghemite ... Reduction of Fe-Ti oxides FE-0-MGO-TI02

SYSTEM

• •

L-79

FeO-Fe203-MgO join FeO-MgO-Ti02V-Ti203) join FeO-Fe203-MgO-Ti02 join FEO-FE203-AL203 CR203-BEARING

L-80 L-80 L-81

SYSTEM

L-Bl

SYSTEMS

L-82

FeO-Fe203-Cr203 system MgAl204-Mg2Ti04-MgCr204 system FeCr204-Fe304-FeA1204 system

L-B2 L-83 L-84

REFERENCES

Chapter 3.

• . . • • • • • • • . •

OXIDE MINERALS

L-84

in METAMORPHIC

ROCKS

Douglas Rumble, III

INTRODUCTION MINERALOGY

R- 1 •

R- I

Spinel solid solutions Hematite-ilmenite solid solutions Pseudobrookite solid solutions Rutile and polymorphs • • • • • • OXIDE MINERALS

IN RELATION

Metamorphic

RRRR-

TO METAMORPHIC

MINERAL

zones of low to intermediate

ZONES

• • . • ••

pressure

• •

Chlorite and biotite zones . Garnet and staurolite zones . . Sillimanite zone . • . . . . . . . Sillimanite-potash feldspar zone Chromian spinel composition in relation to metamorphic High-temperature contact metamorphism High-pressure

metamorphism

OXIDATION-REDUCTION

Chapter 4.

OXIDATION

OXIDATION

zones

R-IO R-IO R-IO R-Il R-II

PROCESSES

R-ll R-12 R-15 R-16

IN METAMORPHISM

R-19

• • . • . • • . • . . • • • • • • •

INTRODUCTION

R- 7 R- 9

R-ll

Element partitioning Oxide mineral equilibria Oxide-silicate mineral equilibria Deduction of conditions of metamorphism

REFERENCES

6 6

R- 9 R- 9

. . • . • • • • • • • • . • . • • •

PHASE PETROLOGY

1 3

of OPAQUE MINERAL

OXIDES

in BASALTS

• • • • •

PARAGENESIS

Primary oxide mineralogy Oxidation of ulvBspinel-magnetite

solid solutions

Trellis type . . Composite types . . . . . . . (v)

R-20

Stephen E. Haggerty Hg-

1

Hg-

3

Hg- 3 Hg- 4 Hg- 4 Hg- 8

Sandwich Oxidation

C4 C5 C6 C?

type

Hg-16 Hg-16 Hg-17 Hg-20 Hg-21 Hg-24 Hg-28 Hg-28

. . . . • . . . . • intergrowths

of titanomagnetite-ilmenite

stage stage stage stage

Oxidation

of discrete

primary

Ilmenite oxidation PHASE CHEMISTRY

ilmenite

classification

OF OXIDATION

Hg-37

ASSEMBLAGES

Introduction Titanomagnetite-ilmenite assemblages Ferrian ilmenite and ferrian rutile . Titanohematite, rutile, and magnetite Pseudobrookite, titanohematite, magnetite OXIDE SYSTEMATICS

AND PHASE

COMPATIBILITY

and Al-magnesioferrite

Hg-37 Hg-46 Hg-49 Hg-50 Hg-52 Hg-55

RELATIONSHIPS

Hg-55 Hg-61 Hg-61 Hg-69

Introduction Ilmenite systematics Titanomagnetite systematics Applications •.••. OXIDATION

Hg-74

Introduction Chromian spinel oxidation Olivine oxidation •

Hg-74 Hg-74 Hg-75

ASSOCIATED

MINERAL

OXIDE DISTRIBUTIONS

IN BASALT

Hg-78

PROFILES

Hg-78 Hg-79 Hg-82 Hg-94

Introduction Mean oxidation numbers Oxide distributions • • Mechanism of oxidation IMPLICATIONS

FOR ROCK MAGNETISM

Hg-95

AND OXIDE PETROGENESIS

Hg-95 Hg-96 Hg-98

Introduction Oxidation • • • • • • Single cooling units SUMMARY

Hg-98

AND CONCLUSIONS

Chapter 5.

OXIDE MINERALS

in LUNAR ROCKS

Ahmed El Goresy EGo- I

INTRODUCTION MINERALOGY

EG-- I

•

Chromite-ulvBspinel Ilmenite-geikielite Armalcolite-anosovite Rutile •••••••.• OXIDE RELATIONS Opaque

oxides

EG- I

series series series

IN DIFFERENT in Ti02-poor

EG-- 3

EG- 4 EG- 4

ROCK TYPES RECOVERED basalts

Spinels •...•....... Cationic relationships and substitutions Cr-Al substitutional trends V-Cr and v-Al substitutional trends

(vi)

FROM THE MOON

EG- 4 EG- 4 EG- 5 EG-14 EG-14 EG-17

Fe-Mg substitutional trends Ti-(V+Cr+Al) substitutions. Opaque

oxides

in Ti02-rich

EG-19 EG-19 EG-24 EG-24

basalts

Armalcolite relationships . Origin of ilmenite rims around armal.coli te in olivine porphyritic basalts Chemistry of armalcolite Chromian ulvospinel Ilmenite . Rutile . Opaque

oxides

in anorthositic

rocks

and highland

breccias

Spinels Ilmenite . Rutile .. SUBSOLIDUS

REACTIONS

EG-38

Subsolidus reduction reactions in the lunar rocks Nature of reducing agent in Apollo 17 basalts REFERENCES

. . . • • . . . . . . . . • .

Chapter 6.

OPAQUE

OXIDE MINERALS

in METEORITES

Ahmed El Goresy EG-47

.

EG-49

Spinel group minerals Ilmenite-geikielite-pyrophanite Rutile . . . . . OXIDE ASSEMBLAGES

IN VARIOUS

EG-49 EG-56 EG-57

series

METEORITE

GROUPS

EG-57

Chondrites

EG-57 EG-59 EG-62 EG-64 EG-65 EG-65 EG-67

Spinels Ilmenite Achondrites

Ilmenite Stony irons Iron meteorites REFERENCES

Chapter 7.

EG-39 EG-42 EG-43

INTRODUCTION MINERALOGY

EG-25 EG-30 EG-34 EG-35 EG-36 EG-36 EG-37 EG-37 EG-37

. . . .

The MANGANESE

EG-71

OXIDES

- A BIBLIOGRAPHIC

COMMENTARY

J. Stephen Huebner INTRODUCTION Crystal

. . . • . • . . . . . . structures

and chemistry

Tetravalent oxides . Trivalent oxides .. "Spine l-type" oxides WUstite-type oxide . Geologic occurrences Thermochemistry and phase relations Petrology of manganese oxides REFERENCES

• . . . . . • . • • . . • .

(vii)

SH- 1 SHSHSHSHSHSHSHSH-

1 I 2 3 4 4 6 8

SH-ll

Chapter 8.

OPAQUE MINERAL

OXIDES

in TERRESTRIAL

IGNEOUS

ROCKS

Stephen E. Haggerty GENERAL

INTRODUCTION

SYSTEMATIC

• • • • . . . • • • • • . • • . • • . . • . • •

Hg-IOI Hg-IOI Hg-I04 Hg-I04 Hg-I04 Hg-I04 Hg-I08 Hg-I08 Hg-I08 Hg-I09 Hg-1l8 Hg-1l9 Hg-123 Hg-128 Hg"'-129 Hg-129 Hg-135

Nomenclature Spine l: series

Ilmenite series Pseudobrookite series Ti02 polymorphs Exsolution . . . . •. Subsolidus reactions . Oxide assemblages

and textures

Two-dimensional mineral morphology Chromian spinelss . Ilmeni te-hemati tess . . . . . . . Reactions involving ilmenite-hematitess Ulvospinel-magnetitess . . . . Titanomagnetite-pleonasteqS . . • . • • Magnetite-chromite-hercyn~tess ..... Reactions involving magnetite-ulvospinelss Oxides derived from silicates PRIMARY

Hg-140

OXIDE DISTRIBUTIONS

Hg-140 Hg-142 Hg-150 Hg-152 Hg-158 Hg-158

Introduction . . • . . . Chromian spinel distributions Pseudobrookite distributions Ilmenite distributions Magnetite-ulvospinel distributions Oxidites .••.•.•••.••• T AND f02 VARIATIONS

IN IGNEOUS

Hg-160

ROCKS

Hg-160 Hg-160 Hg-160 Hg-167 Hg-167 Hg-169 Hg-169

Introduction Extrusive suites Intrusive suites Magmatic ore deposits Data summary Experiment.al determinations of T and f02 •. Silica activity • . • • . . TABLES OF MINERALOGICAL, PETROLOGICAL, AND CHEMICAL OF OPAQUE MINERAL OXIDES IN IGNEOUS ROCKS REFERENCES

Hg-IOI Hg-IOl

MINERALOGY

PROPERTIES

. . • • • . . . . . . . • . • . . . • . • . • • • .

(viii)

Hg-176 Hg-277

The CRYSTAL CHEMISTRY and STRUCTURE of OXIDE MINERALS as EXEMPLIFIED Donald

by the Fe-Ti OXIDES H. Li.nde l es]

Chapter I INTRODUCTION

Crystal structures playa reactions and magnetic

vital role in the interpretation

properties

of the oxide minerals.

of chemical

For most purposes

it

is useful to treat the oxide minerals as ionic crystals that consist of oxygen frameworks

(nearly cubic or hexagonal close-packed)

octahedral or tetrahedral manganese

interstices.

with cations occupying

Both iron and titanium as well as

are members of the first transition metal series; each can therefore

exist in more than one valence state.

Furthermore,

. T'* 3d e 1ectrons In l , Fe U , Fe * , Mn U ,and to these ions.

Thus a complete characterization

mineral must include the determination

the existence of unpaired

. moments Mn * imparts net magnetlc of the structure of an oxide

of valence states and magnetic

tion as well as the position of each atom in the unit cell. is necessary

to chose a magnetic

unit cell that is a multiple

lographic cell, or, alternatively,

to view the magnetic

symmetry than the crystallographic

cell of the same size.

can uniquely characterize mineral, but a combination

orienta-

In some cases it of the crystal-

cell as having lower No one technique

the structure and chemistry of an iron-bearing of methods has yielded detailed

information

oxide

on the

most important structures.

Techniques The primary method of determining fraction.

Single-crystal

the structure,

the structures

is of course x-ray dif-

x-ray studies yield the (non-magnetic)

the positions

symmetry

of

of the metal ions, and with lesser precision,

the positions of the oxygen ions. discovery of x-ray diffraction,

Thus in 1915, only three years after the

Bragg and Nishikawa were able independently

to determine the main details of the magnetite

(spinel) structure.

However,

. Fe 2+ and Fe 3+ to speCl. f'lC sltes . . th e structure th ey were not ab 1 e to asslgn In (the assignment

they assumed is incorrect), nor were they able to determine

the oxygen positions with great precision.

Nevertheless,

the basic structure

provided by x-ray diffraction makes possible the utiiization

L-l

of other

techniques,

for these merely provide

independent

determinations

Neutron

diffraction

is an important

tures of the oxide minerals; It can yield information structures;

similar that x-ray diffraction

titanium

cross-section

in magnitude

netic structure

of magnetic series.

Electrical

magnetic

symmetry

and

large scattering in

them, whereas

Most determinations

of mag-

utilize neutron diffraction. at low temperature

is particularly

can test models

useful in studying solid solution

measurements

and cation distribution;

between

is three times that of

sign.)

magnetization

conductivity

useful.

and thus on magnetic

of iron and titanium

cannot always distinguish

of iron for neutrons

of saturation

the struc-

of iron and titanium are sufficiently

and of the opposite

structures,

in determining

since oxygen has a relatively

and precise oxygen parameters

Measurement

rather than

make it especially

and (3) the distribution

(The electron densities

the scattering

technique

on (1) electron spin orientation

for neutrons;

the structure.

of the structure

three characteristics

(2) oxygen parameters,

cross-section

refinements

of it.

have also been used to predict

this technique has been superseded

for the most part. Naturally

occurring

57Fe in the iron-titanium

oxides permits

of the Mossbauer

effect, which has been particularly

cation valencies,

and, to a lesser extent, magnetic

application

useful in determining structure.

Determination

of the isomer shift for 57Fe in the sample relative

to that in the source pro-

vides a quantitative

measure of the valence

the iron--and,

Magnetic

(or at least semiquantitative)

by difference,

properties

The ensuing discussion cepts of magnetism

and Banerjee

of crystal structures must refer to several

of the solid state.

here; a more extensive

An electron

titanium,

review is given by Nagata

con-

reviewed

(1961, p. 1-39) and by Stacey

in an atom (or ion) has a magnetic

states are self-cancelling.

with one or more unpaired

of both.

from

In iron and

from spin rather than orbital motion. spins but with otherwise identical

Atoms and ions in which all the electrons and are called diamagnetic.

electrons,

and are termed paramagnetic.

tron is one Bohr magneton

moment that results

or from a combination

the moment results mainly

of electrons with opposite

paired have no net magnetic moment

moment

These concepts are briefly

(1974).

its spin, from its orbital motion,

moments

of

that of other cations.

(µB) which

The

quantum are thus

Atoms or ions

on the other hand, have a net magnetic The magnetic

moment of an unpaired elec20 emu. Most para-

equals 0.9274 x 10-

L-2

magnetic particles However,

have but one unpaired electron and therefore have moments

in the first transition-metal

of

series each of the five states

in the 3d level tends to be filled first by a single electron, and to become doubly occupied

only after all five have been filled singly.

as the high-spin

(This is known

When as many 3d electrons as possible

condition.

the atom or ion is said to be in the low-spin condition.)

are paired,

The spins of elec-

trons occupying

the same state must be opposed, in accord with the Pauli exMn2+ and Fe3+, each having five unpaired 3d electrons spins, have magnetic saturation moments of 5µB' Fe2+ has six

clusion principle. with parallel

3d electrons, ·two of which are paired and therefore self-cancelling, resultant saturation moment due to the four unpaired electrons likewise has six 3d electrons a saturation moment of 4µB'

so the Feo

is 4µB'

(as well as two paired 4s electrons) and also has 3 Ti + with one 3d electron has 1 µB' but Ti4+ with

no 3d electrons has no net moment. It might be noted here that the saturation moments differ slightly the susceptibiZity

moments of the same ions.

Susceptibility

moments

from

are some-

what larger, and tend not to be integral numbers of Bohr magnetons. If paramagnetic

atoms or ions are incorporated

with their spins (and hence their magnetic moments) sulting solid has a low magnetic termed paramagnetic.

susceptibility

into a crystal structure randomly oriented,

and no net moment,

the re-

and is

But in crystals of a-iron, for example, exchange inter-

action between neighboring iron atoms results in a parallel alignment of moments

~hroughout

Such crystals

each of several small volumes

that have high magnetic

(permanent) magnetic moments, magnetism

atoms.

are termed ferromagnetic.

is reviewed by Bozorth

Exchange

interactions

In oxide structures,

(1950) provided

however,

the nearest neighbors

oxygens, a mechanism a firm theoretical

Superexchange

approaches

neighbor-metal

parallel

ions are coupled through

basis for superexchange

Anderson

interactions,

of the magnetic

is negative:

ions coupled by superexchange

two equal magnetic

are termed antiferromagnetic

properties

of

angle

the moments

Crystals in which superexchange

L-3

which

of

through an oxygen are anti-

substructures

(Hulthen, 1936).

are

(1934)

becomes more effective as the metal-oxygen-metal

and hence self-cancelling.

actions produce

Kramers

that he called superexchange.

180°, and in general the interaction

~wo paramagnetic

of the metals

cannot be invoked.

now play a central role in most interpretations oxides.

The theory of ferro-

(1951).

that the spins of next-nearest

the intermediate

and can acquire remanent

are strongly effective only between nearest-neighbor

always oxygen ions, and exchange coupling proposed

(domains) of the crystal.

susceptibilities

inter-

with opposite spin directions

A perfect antiferromagnetic

crystal has no magnetic moment, but if impurities, ciencies are concentrated

in one substructure

was postulated,

for example, by Neel

ferromagnetism"

of hematite.

if the spin directions

substructures

are not precisely

perpendicular

to the spin axes; this phenomenon

ature--the Neel point--at destroyed

antiparallel,

substance

Some substances,

and the material

at lower temperatures.

specific heat of an antiferromagnetic

moment

is termed "spin-canting."

there is a characteristic

temper-

structure

is

becomes paramagnetic.

such as ilmenite, are paramagnetic

become antiferromagnetic

of the magnetic

there can be a resultant

and above which the antiferromagnetic

through thermal vibration

as

(1949,1953) to explain the "parasitic

Similarly,

For every antiferromagnetic

defects, or cation defi-

there may be a net moment,

at room temperature

Magnetic

but

susceptibility

material usually reach a maximum

and at or

near the Neel point. For some oxide structures mation of two magnetic structures

super exchange

substructures

interaction

results

in the for-

with opposite but unequal moments.

have a net magnetic moment and were called ferrimagnetic:

(1948), the term being derived netism is best displayed.

from the ferrites

Ferrimagnetic

each have a characteristic they become paramagnetic;

temperature--the

is sometimes

are closely similar

Curie point--at

conveniently

materials.

Thus it is common to speak of the "ferromagnetic mineral,

even t~ough those properties

netism.

Both ferrimagnetism

properties"

would exist even in a perfect

of a rock or

are almost certainly due to ferrimag-

and antiferromagnetism

the inequality

properties

and the former

a subclass of the latter.

with parasitic

netism result from two opposite but unequal magnetic ferrimagnetism

and above which

Many macroscopic

considered

ones,

the Curie point

to those of ferromagnetism,

if inelegantly

ferrimag-

like ferromagnetic

there is a close analogy between

and the Neel point of antiferromagnetic of ferrimagnetism

by Neel

(spinels) in which

substances,

Such

substructures,

is a result of the crystal structure

ferromagbut in and thus

crystal.

THE CUBIC OXIDE MINERALS

Many oxide minerals

have a cubic structure;

others are perhaps

viewed as slight distortions

or modifications

general the oxygens approach

cubic close packing.

of isometric

sites.

If only octahedral

spinels--both

octahedral

L-4

In frame-

sites and 32 octahedral

sites are occupied by divalent cations,

(MgO), the crystal has the NaCl structure

group of oxides--the

structures.

A cubic-close-packed

work of 32 oxygens contains a total of 64 tetrahedral

clase

best

as in peri-

(Fig. L-l). In a very important

sites and tetrahedral

sites

oo Q

*

Oxygen Octahedral Origin

Figure L-I. A view of the monoxide structure,

emphasizing

cations

(periclase, manganosite,

the alternating

ideal wlistite)

(Ill) planes of oxygens and cations.

Eight unit cells are shown to facilitate comparison with the spinel structure (Fig. L-3).

-----------------are occupied.

Not all of both sets of sites can be filled, however,

would require face-sharing between octahedra and tetrahedra--which getically unstable because of electrostatic filling of 8 tetrahedral sites; conversely,

for this

is ener-

repulsion of the cations.

The

sites precludes occupancy of more than 16 octahedral

the filling of 16 octahedral

only 8 of the 64 possible tetrahedral

sites permits occupancy

of

sites, if the symmetry of the space

group is maintained.

Monoxides

(Space group Fm3m)

Like periclase, manganosite valent cations occupying

(MnO) has the NaCI structure, with the di-

the octahedral

interstices.

axis, the structure consists of alternating (Fig. L-I).

L-5

Viewed along the [Ill]

(Ill) layers of oxygen and metal

-x ()5 .10 4.33,Or-----,------,---,-----, Kotsuro et 01. (1967)

.0 /-

//-

////-

Figure L-2. Unit cell dimension

///-

of wlistite as a function of

/-

composition.

1.00

0.95 I-X

Fe/O (atomic)

Although

rare as a mineral

(but see, for example, Walenta,

phase wlistite (nominally FeO) is of considerable troversy raged over whether

stoichiometric

agreement that stoichiometric

FeO exists.

the replacement of 3x Fe U by 2x Fe

* , so

or, more simply, Fel_xO.

one atmosphere

is about Fe .

a range from 4.309

A

1960), the

For years a con-

There is now general

FeO cannot exist as a stable phase at low pres-

sures and that it is always cation-deficient.

2+ 3+ Fel_3xFe2xO,

interest.

0

Charge balance

is maintained

that the formula may be

. wrltten

by

The most iron-rich wlistite stable at

(Darken and Gurry, 1945).

O 954 for Fe . 0 to 4.292 O 949

X

Lattice

constants

for Fe .9140, the exact values O (Foster and Welch, 1956); see

depending on the thermal history of the samples Fig. L-2. Katsura et al . sures above 36 kb.

(1967) reported synthesis of stoichiometric The value a = 4.323

close to that predicted

±

A

0.001

determined

for pure FeO by extrapolation

(4.326

FeO at pres-

by them is very

X).

Roth (1960) has made a detailed study of the defect structure by neutron diffraction. of vacancies

The diffraction

in the octahedral

iron positions,

than are required by the chemical analysis. tion data indicate 0.08 vacancies Fe .

0.

Evidently

but indicate more such vacancies

For example, the neutron diffrac-

per octahedral

iron site for the composition

0.02 iron ions per formula unit must be accommodated

O 94 tetrahedral sites interstitial

symmetry is maintained is F'd3m.

of wlistite

patterns not only confirm the presence

to the close-packed

if the unit cell is doubled;

oxygen framework. the resulting

space group

The symmetry and oxygen content of such a cell are identical

L-6

in

Cubic

to that

of magnetite, and, indeed, Roth visualized the local structure in the vicinity of octahedral vacancies as essentially that of magnetite. Roth's model has been generally confirmed by single crystal x-ray diffraction studies (Koch et al., 1966; Koch and Fine, 1967, Koch and Cohen, 1969) which suggested the presence of ordered complexes of octahedral vacancies and tetrahedral cations.

Shirane et al. (1962) have found Mossbauer evidence that the 2 local symmetry around the Fe + irons is less than cubic; they infer that the

lower symmetry is due to the vacancies.

Some studies have shown that quenched

wlistites may have one of three slightly different structures (Vallet and Raccah, 1965; Kleman, 1965; Carel, 1967).

Swaroof and Wagner (1967) could find no evi-

dence of phase changes in the wlistite field at 950-1250°C; however, high-temperature x-ray diffraction studies by Manenc (1968) show that at least one of the ordered structures persists to 1000°C for Fe-poor compositions.

Manenc sug-

gests that at temperatures within the wlistite stability field, Fe-rich wlistites have the true NaCI structure (with the vacancies random) whereas Fe poor wlistites have an ordered structure.

Spinel group A large number of oxide minerals, some sulfides, and many artificial substances crystallize with the spinel structure, which is extraordinarily flexible in terms of the cations it will accept.

At least 30 different elements, with

valences ranging from +l to +6, can serve as cations in oxide spinels.

Some

geologically important spinel end members are listed in Table L-I. Of the irontitanium oxides, the magnetite-ulvospinel solid solution series has the spinel structure, as do the cation-deficient oxides--at least as a first approximation-maghemite and titanomaghemite.

The unit cell is face-centered, cubic, and in

oxide spinels contains 32 oxygens, which form a nearly cubic-close-packed framework as viewed along the cube diagonals ([Ill]), the space group being Fd3m. The cations occupy interstices within the oxygen framework (Fig. L-3). In space group Fd3m there are two alternative sets of compatible tetrahedral and octahedral sites:

16d

(octahedral) and Sa (tetrahedral) or l6c and 8b (International

Tables, 1952, p. 340-341).

These sets, although mutually exclusive, are iden-

tical in that a translation of the origin by ~, ~, ~ will bring one set into co~ncidence with the other. the spinels.

By convention, the set 16d and Sa is chosen for

There is considerable variation in the literature regarding the

notation used to describe these sites.

Most workers have employed the Wyckoff

(1922) notation, others that of the International Tables--which is followed here--and still others a notation based on the concept of magnetic substructures

L-7

o o

o

*

Oxygen Octahedral Tetrahedral

cations cations

Origin

Figure L-3. The spinel unit cell, oriented so as to emphasize the (Ill) planes. Atoms are not drawn to scale; the circles simply represent the centers of atoms.

The origin in this diagram lies at the center of symmetry, as recom-

mended by the International Tables (1952); it differs by (~,~,~) from the origin used in much of the literature.

The arrow at the top indicates the

cation and oxygen layers shown in Figure L-5.

L-8

Table L-l. Some spinel end members.

Mineral Name

Cell. EdRe a, 1n

Formula Fe3+[Fe2+Fe3+]04 Fe3+[Mg2+Fe3+]04 Fe3+[Mn2+Fe3+]04

Magnetite Magnesioferrite Jacobs ite Chromite Magnesiochromite Spinel Hercynite Ulvllspinel

Fe2+[cr~+]04 Mg2+[Cr3+]O M 2+A13+0 4 9 2 4 Fe2+[A13+]04 Fe2+[Fe~+Ti4+]04

Oxygen Parameter Struc ture

u

8.396 8.383 8.51 8.378

0.2548 0.257

8.334 8.103 8.135 8.536

0.260 0.262

N N 7/8 N

0.261

X[Y J0 , where [ J indicate octa2 4 I, "inverse" distribution, Y[XY]04' Many spinels

N, "normal" cation distribution, hedral cations. are probably (1970,

A and B.

p.

intermediate

between

these extremes.

Data from Burns

110).

For the convenience of those who wish to pursue the original litera-

ture, TableL-2 correlates these various notations.

Table L-2. Nomenclature

Tetrahedra 1

of cation sites in spinels.

Octahedral

Example of Usage Wyckoff (1922, 1965) N~el (1948) International Tables (1952)

f

c

A

B

a

d

The sites occupied by cations in the spinel unit cell are special sites; that is, they lie at the intersections of symmetry elements.

These sites are

therefore fixed, and their coordinates are given by rational fractions of the cell edge, a.

Thus the coordinates x, y, z of one tetrahedral (a) site are

~ a, ~ a, ~ a (abbreviated ~, ~, ~).

The coordinates of all the sites are

given in Table L-3,based on an origin at the center of symmetry, rather than the more usual origin at - ~, -

i, -~.

The 32 oxygens, on the other hand,

occupy sites whose exact coordinates must be determined experimentally.

Only

one parameter, conventionally called u, needs to be determined; the oxygen

L-9

Table L-3. Coordinates of anion and cation sites in sn i ne l s . Space group Fd3m. Origin at center (3m).

32

Anion

Coordinates

Symmetry

Nota ti on

No. of Sites

Type of Si te

1 u,~-u,

u,u,u;

3m

e

1 1 ~-u'4-u,u; 3 4+ Tetrahedral Cation Octahedra 1 Cation Note:

1

1 ~-u;

----3 u,u,u;u'4

- 3 u, u'4+ 1

1

1 1 4-u,u,~-u;

7

3 u; 4+ 7

3 + u, 4 + u; 3 4+

U,

7

8

a

43m

8' 8'

16

d

3m

111. 111. 111. 111

Because of the face-centered

lattice,

u, u

8; 8' 8' 8

2'2'2'

2'4'4'

4'2'4'

4'4'2

there are four points equivalent

to the origin:

The full sets of sites are generated table to each of the equivalent Reference:

coordinates

International

parameter u happened correspond

cube diagonals,

Variations

cations in the tetrahedral

framework

considerations.

The oxygens lie in

to equal ~, the oxygens would occupy special sites which Inasmuch as u does not deviate

and useful approximation

in u correspond

and reflect adjustments

to an enlargement

in the

(1952, p. 341).

to rigorous cubic close-packing.*

as close-packed.

the coordinates

in terms of u are given in Table L-3. If the oxygen

greatly from ~, it is a reasonable

diminution

Tables

are then derived from symmetry

32 e sites whose coordinates

by applying

points.

to displacements

of oxygens along

to the relative effective

and octahedral

of the tetrahedral

in the octahedra

to view the oxygens

sites.

An increase

coordination

polyhedra

(see Fig. L-4). In addition,

radii of

in u corresponds

and a compensating

the entire oxygen

(i.e., the unit cell) can expand or contract to accommodate

of larger or smaller average effective

radius.

It is this flexibility

cations of the

(-i,

*Many workers choose an origin for the spinel structure at ~3m, which is -~, -~) removed from the origin at the center (3m) adopted here. The values for u listed here should be increased by 0.125 to compare them with most values in the literature.

L-10

u

decreasing

Figure L-4.

Details along a body diagonal of the spinel unit cell in Fig. L-3,

illustrating

how changes in the oxygen parameter u change the relative

the:

""""" .......... ~l-.~....1 ...... ~1 ,-C'-J...Cl.llCUJ..O-..L

~_...J o.tlU

oxygen framework

__ .... _l..._.J U"-LaLLCl.lJ..a..L..

,

_.:

....

sizes of

__

;:'.J..Leb.

that permits such a large number of elements

to occur as im-

portant cations in oxide spinels. The general chemical formula for ideal spinels is XY 0 (or X8Yl6032 per 2 4 unit cell), where X and Yare cations of different valence. In magnetite X 2+ 3+ 4+ 2+ Fe and Y = Fe ; in ulvospinel, on the other hand, X = Ti and Y = Fe . The original determinations 1915) were made on magnetite

of the spinel structure (Bragg, 1915a,b; Nishikawa, 2 and on spinel (X=Mg +, Y=AI3+); it was impossible

to distinguish

between the X and Y cations on the basis of x-ray intensities ". U H because of the slmllar scatterlng powers of Mg and Al and of Fe and Fe . For simplicity

that the eight X cations occupy the 8a sites and

it was assumed

the sixteen Y cations occupy the l6d sites, giving a structural X[Y2]04' where and Posnjak

the brackets denote cations in octahedral

(1931,1932) discovered

sites, yielding

spinels, but

The latter have 8 of the 16 Y cations in the 8a

the structural

formula Y[YX]04'

termed these spinels "inversed," spinels."

Barth

by careful x-ray intensity measurements

that this "normal" structure was correct for several aluminate not for many other spinels.

formula:

(16d) sites.

and at present

Verwey and Heilmann

It is now known that most spinels are intermediate

L-ll

(1947)

they are known as "inverse between

these

two extremes.

Table L-I indicates which end member spinels are normal and which

are inverse. Verwey and Heilmann (1947) also gave empirical rules regarding the distribution of cations in the tetrahedral and octahedral sites of the spinel structure.

Electrostatic and ionic radius considerations alone are insufficient to

explain the observed distributions.

Goodenough and Loeb (1955) showed that 3 cations having a tendency to form hybrid sp3 bonds--such as Fe +--are favored in tetrahedral sites.

There have been suggestions that ions thus bound in

tetrahedral sites are relatively immobile (Frolich and Stiller, 1963; O'Reilly and Banerjee, 1966), but self-diffusion experiments indicate otherwise (Lindner and Akerstrom, 1956). Magnetite

(Fe304J•

Both Bragg (19l5a,b) and Nishikawa (1915) determined

the magnetite structure using Laue x-ray photographs obtained from single 2+ 3+ crystals. They assumed that the structural formula was Fe [Fe2 ]04, and that the oxygen parameter u was 0.25, although they recognized that it would vary with cation composition. O.OOI.

Claassen (1926) refined the value of u to 0.254 +

Hamilton (1958) further refined u to 0.2548 + 0.0002 at 23°C from

neutron diffraction data. A wide range of values for the unit-cell parameter (a) of magnetite has been reported.

Several reasons for this variation exist.

Many of the magne-

tites were not adequately characterized chemically, and the discrepancies probably reflect the presence of cations other than Fe2+ and Fe3+, or of cation vacancies.

Precise x-ray work on natural and synthetic magnetites of known

A.

composition has yielded values of a ranging from 8.393 to 8.3963 The similarity in scattering power of Fe2+ and Fe3+ made it impossible for the early workers to determine the structural formula of magnetite by x-ray methods.

Verwey and deBoer (1936) used measurements of electrical conductivity 2 3 to demonstrate that magnetite has the inverse spinel structure, Fe3+[Fe +Fe +]04·

Neel (1948) predicted that the irons in the 8a and the 16d sites form two magnetic substructures with antiparallel moments along [Ill].

If magnetite had

the normal structure, Fe2+[Fe~+]04' there would be 8x4 µB in the 8a sites and 8x(5+5) µB oppositely directed in the l6d sites, for a net moment of 48 µB per unit cell [=6 µB per formula unit]. In the case of inverse structure, Fe3+ 2+ 3+ [Fe Fe ]0

. the net moment would be 8x(4+5)-8x5 = 32 µB per unlt cell [=4 µB 4 per formula unit]. The empirical value (extrapolated to OOK), of 4.07 µB (Neel, 1948, p. 179) per formula unit thus supports the Neel model regarding both the inverse structure and the ferrimagnetic structure.

Neel assumed that

the excess moment (.07 µB per formula unit) reflects an orbital contribution L-12

2

to the moment completely

of Fe

diffraction

[Ill] axis

It is along is observed

to view

planes

complex

because

planes:

normal

and that

of iron ions.

tetrahedral

the structure

was provided

the magnetite

this axis

In the wlistite structure

with

is that

of the model

is not

by the neutron

et al. (19Slb).

(Fig. L-5).

ternate

between

of Shull

it is useful

of the oxygens

explanation

confirmation

purposes

be directed.

where

an alternative

Final

experiments

For many

packing

+;

inverse.

the ferrimagnetic

normal

and octahedral

the sequence

along

cubic

moment

to [Ill], planes

In magnetite

to [Ill] the sequence

structure

that the (nearly)

the

closetends

of oxygens

is somewhat

to al-

more

cations do not lie in the same VI IV VI IV VI -O-Fe -Fe -Fe -O-Fe

is O-Fe

the superscripts refer to coordination numbers. The distance IV VI Fe layers and the adjacent Fe layers is small (0.675

along

A,

a ..

o

[111]

or 0.08 a),

Octahedral

Tetrahedral

1A

f-----j

Figure

L-S.

spinel

and the cation

The

large

are drawn

Diagram,

approximately layers

open circles

layer,

(al3).

Note

close-packing,

expressed

on either

are oxygens.

for magnetite.

hedral

to scale,

Decimal

the height

plane

an oxygen

side of it, projected

The ionic fractions

as a portion

that the oxygen

showing

radii

show height

of the body is puckered

of the oxygens

would

L-13

(Shannon

diagonal

slightly;

above

layer

onto

of a

a (lll) plane.

and Prewitt, the lower

of the unit for ideal

be 0.144 (= 1~13).

1969)

octa-

cell

cubic-

and for some purposes

it is a good approximation

to consider

successive planes of O_FeVI_O_(FeIVFeVIFeIV)_O_FeVI

....

netite as viewed in this way bears many similarities the (Ill) planes play an important

the structure as

The structure of mag-

to that of hematite,

role in the textural relations

and

between these

two minerals. The Curie temperature range ferrimagnetic peratures.

of magnetite

Values of the Curie temperature

been reported.

The transition

thermal expansion,

to a transition to disorder 0

ranging from 570

is accompanied

by maxima

the specific heat and the neutron

At low temperatures

magnetite

a decrease in crystallographic properties.

corresponds

ordering at lower temperatures

Probably

undergoes

symmetry,

synthetic

o

to 58l C

have

in the coefficient

scattering

of

cross-section.

another transition which involves

electrical

conductivity,

the best value of the transition

obtained for stoichiometric,

from long-

at higher tem-

single crystals

temperature

and magnetic is 119.4°K,

(Calhoun, 1954).

8.540r------,-----,------,------,------~-----,------r-----_,------------,

0

....

:z .' .....

with composition in the (l-x)Fe304xFe2Ti04 series. Dotted line, predicted

c OJ

E

0

~ c

values for Akimoto's (1954) model.

2

Dashed line, predicted values for

0

.~

the Neel (1955) and Chevallier-

::J

-0

if)

;/

Bolfa-Mathieu (1955) model.

Solid

line, predicted values for the O'Reilly-Banerjee (1965) model.

°0

.

Stephenson (1969)

.to

Bleil (1971)

Banerj ee (1965)."

F 3+[F 2+ F 3+ T·4+]02e el x el-2x lX 4

o

3+ 2+ 2+ 3+ 4+ 2Fel.2_xFex_0.2[Fel.2FeO.8_xTix ]04

0.2 < x < 0.8

3+ 2+ 2+ 4+ 2Fe2_2xFe2x_I[Fe2_xTix ]04

0.810 -0.68

in Fe Ti0 . Z 4 in the phase

for 11m-Hem

±

solubility

of FeTizOS

perature.

Ranges

cible regions

Ilmenite

±

(FBb) in FeZTiO

of complete

(Pb) increases with increasing temS miscibility are shown as solid joins and immis-

are dashed.

systematics

The progressive along

immisCibility

the oxidation

of assemblages

oxidation reaction

according

of stoichiometric isopleth,

ilmenite

will pass through

to the phase relationships

at 9000e, proceeding the following

sequences

shown in Figure Hg-14a:

(1) Ilmss (i.e., partially enriched in Hem ) + R; (Z) Ilmss (now more highly ss enriched in Hem ) + R + Pb ; (3) Pb + IlmHem ; and (4) Pb + R. At S8 S8 S8 S8 S8 8000e the two-phase field R + Ilmss and the three-phase field Ilmss + R + Pb ss are expanded proportionately and the assemblage sequence is now: (1) Ilmss

+ R; (Z) Ilmss + R + Pbss; and (3) Pbss + R. These expansions 700° and at 6000e because

of the rapidly

contracting

Pb

the 11m-Hem

ss This marked

original

sequence

miscibility gap is encountered. ss three-phase regions and the oxidation

continue

at

limit and because change

splits

the

(1) Ilmss + R;

is:

(Z) Ilmss + Hem

+ R; (3) Hemss + R; (4) Pbss + Hemss + R; and (5) Pbss + R. ss of basaltic composition the only likely changes in the reaction are for oxidation at 9000e where the onset of oxidation will yield

For ilmenites sequence Pb

coexisting with R + Ilmss (see Table Hg-Z). Stabilizing minor element ss components or ilmenites with initially larger proportions of Fe 0 in solid Z 3 solution will influence the rate at which oxidation is initialized, but the

progressive

sequence

Titanomagnetite Spinel tion because temperatures. ponent

is unlikely

to differ

in overall

format.

systematics

oxidation

is abundantly

of the associated However,

of the assemblage

more complex

production

if chosen

to be viewed

will oxidize

ilmenite

separately,

in parallel

Hg-61

than that of ilmenite

of "exsolved"

oxida-

at sub solidus

the ilmenite

to and will conform

com-

to the

Figure Hg-14.

(a-d) Phase compatibility

data for Ilm-Hem

(Lindsley,

ss

series

(Haggerty

diagrams

constructed

1973) and the decomposition

from the solvus

data for the Pb

1970) at 600°, 700°, 800°, and 9000e.

and Lindsley,

ss Tie lines

indicate the compositional limits of possible coexisting phases. The Fe 0 3 4 Fe Ti0 series is complete at these temperatures; Ilm-Hem is complete at z 4 ss approximately 8000e and is taken as complete in (c); the Pb is incomplete for ss the temperatures considered and the small but distinct limit of solid solubility of FeTiZOS sequence

should be noted for the 6000e section (a). The S discussed in the text are obtained by moving across

in FeZTiO

of assemblages

each of the ternary

diagrams

(dashed) from the TiOz-FeO (e) Oxide assemblages each of the stages

along oxidation-reaction

join towards

relevant

lines of constant

Fe:Ti

the TiO -Fe 0 z z 3 join (see Fig. Hg-13). to each of the appropriate phase regions for

of oxide oxidation.

Hg-6Z

MOLE

PERCENT

Figure Hg-14(a).

Hg-63

Ti02

B

Pbss

FeO

Fe304 MOLE

PERCENT

Figure

Hg-14(b).

Hg-64

+ Hem .. + R

c

FeO

Fe304 MOLE

Figure

PERCENT

Hg-14(c).

Hg-6S

FeO

Fe304 MOLE

PERCENT

Figure Hg-14(d).

Hg-66

C2

C4

MOLE

C5

PERCENT

sequence

systematics

depleted

titanomagnetite

minor

R + Pb

associated

limited

defined

then relatively

and a sequence

equations

oxidize

listed

ss large proportions

while

the residual

to titanohematite if subsolidus

of USPss will remain

can be evaluated sequences

with perhaps

"exsolution"

which are not too far removed

Both processes

and Ti-

is

in the spinel

from that of il-

from the phase are considered

compatibi-

in the

in Table Hg-S. typical

case in which

oxidation

exsolution

at 9000e for an ilmenite

of reactions

;

does take place,

with ~90 mole % FeTi0

3

+ Ilm-Hemss; ss

(3) Pb

is:

+ R. ss

and (4) Pb

ss , the two-phase R + Ilmss assemblage is likely to be 90 but the remaining sequence will hold. With progressively decreasing

For values

of 80% of titanomagnetite

and the sample is therefore

data for extreme

both ferric iron and magnetic

For extreme

homogeneous)

that average

magnetic

bulk chemical

ample demonstration

obvious

level were established

which was present

containing

classification

at the Rl stage,

It> soon became

alizing

a sample

(i.e., optically

state

and in the revised

attempts

of the mean oxidation

oxide assemblage

For example,

also be homogeneous ClRl.

estimates

the dominant

of

oxide classification in terms of phase data by Lindsley;

and the third is the modal determination counting

Mean

oxidation

determinations

±

to within

in 2.5 cm diameter

observers

sections

depends

or displays

on variations

original

by 12

of grains

a sample

With experience

oxidized

a decision

ilmenite:

in distinguishing

is fol-

between

grain morphology

(1)

and ilmenite

skeletal

can

observations.

if the oxide classification

are employed

and original

of

is uniformly

crystal

and

habits;

of R:Hem

and of Pb:Hem (these ratios are larger in oxiss ss (3) relic {Ill} planes; and (4) residual rods or blebs of

dized ilmenite); spinels

criteria

in titanomagnetite

the ratios

The number

grain counts or for multiple

is rapid and straightforward

titanomagnetite

based on determinations

and on whether

in assemblages.

for additional

the distinctions

esti-

zOO-SO~ oxide grains

in grain size, on the ratio or abundance

a wide disparity

lowed and if the following

black

point

with a mean grain size of 50 µm and

(statistics

of the same sample).

and of ilmenite,

quickly

The technique

(2)

classical

of oxide assemblages,

for counts of between

for crystals

and 22 observations

titanomagnetite

be made

of the distribution

5% are possible

of 5-10% by volume

a concentration

counted

using

numbers

In modal mates

of assemblages

techniques.

(pleonaste-magnesioferrite)

are indicative

of original

titano-

magnetite. Mean

titanomagnetite

the percentage tion.

and ilmenite

of grains which

This value

(for ilmenite)

numbers

are determined

fall into each of respective

is expressed

according

oxidation

either

stages

as MC (for titanomagnetite)

by

of oxidaor as MR

to:

MC

where MC or MR

mean oxidation

Cl to C7

titanomagnetite

Rl to R7

ilmenite

Apart

measures

from the decision

dances

which

or originally

lamellae

of whether ilmenite,

class.

an oxidized

grain was originally

the only additional

are evaluations

in titanomagnetite

site or sandwich

intergrowths

abundance

once again of ferrian

levels

stages

stages

in each oxidation

should be exercised

of ilmenite

oxidation

oxidation

% of grains

tanomagnetite

number

are primary

depend

on the abun-

(CZ or C3), on whether

or oxidation

rutile

Hg-79

which

products,

in ilmenite

ti-

precautionary

compo-

and on the

(R2 or R3).

As a

Figure Hg-IB. based

Oxidation

on the assemblages

profiles

across

of oxidized

14 single lava flows from ICeland

titanomagnetite.

The thinner

flows AS

to DS are flow units and the remaining

flows have clearly-defined

lower chilled

margins.

MS are from the same flow sampled

approximately

100 m apart,

Flow HS which tion indices in the text. another,

Profiles

and are shown in greater

is the thickest are listed

Sand

flow is discussed

from 1 to 6, where

Note that the maximum

and vary also as a function

Pbss + Hemss

(6 or C7) show preferred

flows or maxima

detail

levels of oxidation

at one third or two thirds

Hg-BO

towards

The oxida-

to C7 as defined

vary from one flow to

of flow thickness. maxima

at

in Figure Hg-19.

in Figure Hg-ZO.

6 is equivalent

upper and

Those flows exhibiting

the central

from the base.

portions

of

lJ1

f-'

ex>

OQ I

I

~ ZS% by area) R3 and C3.

compositional

or small numbers

information,

C3 with the appropriate example

as CZ

studies

suffixes

(composite),

it is imverative

oxidation

of olivine

olivines

denoting

which display

composite

oxidation

and sandwich

independent as Cl, CZ or types, for

or C2cs if both are present

of trellis

ilmenite

also that some estimates

olivines

require

should be classified

CZs (sandwich),

and close, attention

to oxidized

are C2 and RZ and large

intergrowths

and these grains

c with a small number

association

of lamellae

Composite

lamellae.

in

For magnetic

be made of the level of

given to the ratiqs of unoxidized

and of the ratios

of Fe 0 :Fe 0 3 4 z3

in crystals

(see Fig. Hg-17).

Oxide distributions Three examples blages within

are given to illustrate

single

cooling

(Watkins and Haggerty, Makaopuhi

lava lake

of oxidation basalts

restricted blages

panying

1960; Lindsley basalts

and Haggerty,

because

the major

and figure

The oxide distributions

1971).

attention

equivalent

subtle

The discussion

These distributions

across

and those

based on a scale of 1 to 6 where to C7, and the distinctions

1-5 are between

Pb

) and C7 (Pbss+Hem ) as defined here are not shown. This ss ss discrimination is important but does not affect the overall trend of or the positions

of maximum

tions and intensity

of magnetization

are shown in Figures

19c-d, respectively, 100m apart.

for profiles The. central

Sand

of the concentration R6 and R7.

zone of high oxidation

of titanomagnetite

The oxidation

of Ilmss and Mtss

are now compared with

profile

a good correlation

(Fig. Hg-19c),

Oxide Hg-19a-b

Hg-1ge

distribuand Hg-

at approxi-

at approximately and Hg-19f

closely parallel the intensity

among parameters

at stages

each other,

and

of magnetization is apparent.

(1) The rise and final peak in maximum Hg-8Z

2m

as functions

at stage C7, and of ilmenite

if these distributions

should be noticed:

oxidation.

MS which were samples

above the base of the flow are shown in Figures

points

is

lavas, with two profiles

oxide distributions

mately

are

in the oxide assem-

are self explanatory.

14 single

to Cl to C5, 6 is equivalent

C6 (incipient

Gorge

The examples

should be given to the accom-

one lava (S and MS), are shown in Figure Hg-18. that follow are for titanomagnetite

basalts

and for zones

Picture

wide variations

captions which across

1971),

from the Oregon

and dikes are known to be limited.

brief because

diagrams

are for Iceland

1966; Haggerty,

to joint selvages

to extrusive

of plutons

necessarily

The examples

in oxide assem-

et al., 1968), a drill core from the

(Sato and Wright,

adjacent

(Lindsley,

units.

1967; Wilson

the variations

Two

oxidation

is relatively

sharp when approached

progressively

on the upper side, a situation

(Fig. Hg-19d);

and

is not reflected decomposition

distribution

olivine

profile

an extrusion

of the flow on an underlying Watkins

and Haggerty,

models

the distribution

profile,

of oxidation

a result

of Mtss

S after a period

of 1000°C and deuteric

and a free-air

cooling

upper face (Jae~er,

compatibility

assemblages

MS

In Figure Hg-19g

is shown for traverse

The phase

in profile

at the base of the flow

decomposition.

temperature

basalt

1967).

is reversed

of magnetization

of only partial

assuming

which

in oxide oxidation

in the intensity

but

the temperature of 58 weeks

(2) the maxima

from the base of the flow but falls off

ternary

1961;

at 6S0°C closely

with two and three phase

assemblages

on the Fe203-rich portion of the diagram being typical of the high oxidation region, and with two phase assemblages on the FeO-

central

rich portion

of the diagram

being

typical

of adjacent

relatively

unoxidized

zones. Magnetic thickest

and oxide parameters

flow shown

(HS) , which

is the

are given in Figure Hg-20a-c.

These

et al. (1968), and in this example the peak in oxidation

data are from Wilson is at approximately once again,

for a second flow

in Figure Hg-18,

Sm from the base of the flow which

intensity

of magnetization

and with

the oxidation

markable

constancy

of olivine.

correlates

maximum

grain sizes for ilmenite

dation,

and for titanomagnetite

which which

of interest

and of the relative correlates

Here,

with high oxide oxidation

Other features

of Curie temperatures

is 16m thick.

are the re-

positions

of

with the zone of high oxi-

is at a maximum

at 9m above the flow

base. The major

points

to notice

The state of oxidation states of oxidation although

in many

the base;

for the Iceland

is highly variable

are present

cases maximum

(3) the Fe-Ti

towards values

oxidation

ratios;

(4) grain size variations

blages;

(5) traverses

of maximum oxidized

oxidation;

The example

and values

the flows;

(Z) maximum

of the flows

are either one third or two thirds from

index

correlates

parallel

and (6) magnetic intensely

(1)

positively

the distribution

property

magnetic

with FeO:Fe 0 2 3 of oxide assem-

in the positions

measurements

show that highly

and more stable magnetically

than

zones.

the distribution

Wright

throughout

are as follows:

the central portions

from the same lava show differences

zones are more

unoxidized

basalts

from the Makaopuhi of an oxidized

of fOZ were measured

(1966).

Oxide parameters

lava lake is shown in Figure

zone between

5500 and 7S0°C.

Hg-Zla

for

Temperatures

in situ, and the data are from Sato and as a function

Hg-83

of depth and of FeZ03:FeO

for

Figure

Hg-19.

ness.

The profiles

equivalent

(a-b) Oxide distributions are approximately

(1967).

(c-d) Intensity

for the same suite of samples dence between

high intensities

the base of the flow which not of olivine.

S.

magnetization

is a reflection

of oxides

and it is of interest

I-V are

J (emu/grn x 10-3)

with a close

corresponis at

of the oxides but

at each of the oxidation

from the zone of maximum

bear a much stronger

indices

The only discrepancy

of the oxidation

percentages

of thick-

Data from Watkins

magnetization

and high oxidation.

(e-f) Relative

profile,

of natural

Oxidation to C7.

shown in Figure Hg-19a-b

C7 and R6, R7 for samples

These distributions

MS as a function

100 m apart.

to Cl to C5, and index VI is equivalent

and Haggerty

stages

from Sand

oxidation

relationship

in profile

to the intensity

of

+ ss

to note that C7 and R7 (Pb

Hem

after titanomagnetite and after primary ilmenite, respectively) are ss virtually identical. (g) The curve is the polytherm after approximately one year for a lava of 10 m in thickness 1000°C

on

an underlying

of the polytherm

which

rates of convective

basalt

slower at the base because

lava.

The polytherm

cussed

in Watkins

compatibility

clearly

relationships

that the lower central this region

temperature

The assymetry

S results

from the relative

effects

from the data by Jaeger

(1967).

The ternary

+

oxidation

surface

but rela-

of the underlying (1961) and is dis-

shows the expected

at approximately

by Mtss

of

upper face.

is rapid at the free-air

for the oxides

the zone of maximum

it is within

oxidation

is calculated

by profile

of the insulating

of the flow are characterized

and within

hence

ideally

heat loss which

tively

portions

at an initial

and with a free-air

is modeled

and Haggerty

extruded

Ilm

650°C.

' below

ss

+ Hemss ss

by Pb

phase

The upper

this by R

The polytherm

+

Hem ss shows

third of the flow cools at the slowest

rate, and

that volatile

and high

ensues.

Hg-84

accumulation

takes place

PROFILE S

PROFILE MS

,-

10

I

I

I

0

I

~ J

Z5

I

75

I

I

I

I

Meters

;

Meters

I

I

I

I

I

2.5

I

I

I

I

I

I III

IV

Oxidation

Index

V

I VI

III

Oxidation

(a)

PROFILE MS

10f--f---

101==

I--

15t

rI-

" r--Meters

Index

(b)

PROFILE S

I=--

5

'1

.5

~-

~ J

J

(C)

(d) Figure

Hg-19(a-d).

Hg-85

IV

VI

PROFILE

S

7·5

TITANOMAGNETITE

(/)

a::: ~ 4·5 w ::!;

lJ1

.....

I 00

()Q

'>j

".

OQ

~'" ~

OQ I I-'

lJ1

(1l

"

c

OQ

Meters

2.5

5

7.5

200

in

400 Temperature (g)

300

'c

500

600

700

Figure Hg-ZO. HS.

Sample

(a-b) Magnetic

and oxide parameters

1 is at the base of the flow and sample

are from Wilson mum oxidation

et al. (1968).

showing

(c) Fine-scale

a close correlation

for 30 samples

from profile

30 is at the top.

definition

among parameters

Data

of the zone of maxiexcept for high

Fe 0 :FeO ratios at the base of the flow. Note the surprising correlation of Z 3 ilmenite grain size and that the maximum grain size for titanomagnetite is 3 m higher

in the flow (refer also to Fig. Hg-19b).

!~::l··......__..__..-O-,......_...__._-------------~-----....--..,....-. 0::

:J

U

!:;8ffi w z

200

OJ_--------------------------------------------

:1

~j~~.~ > ,_o:: ::10 _,_ IDU

fA~

•o

~~f1~ 111: 12

o

J

t,

5 6 1

8

9

10

4

11

12

lJ 11.

6

IS

16

T1

1&

19

20

8

POSITION

21 22

10

IN

23

210 25

12

LAVA(MEffiES)

(al

Figure Hg-ZO(a). Hg-89

26

XI

14

21 ~

]0

16

SPECDEN N.J.4BER

10

oL------------------------------------------200 UJ N

iii

z

-.j

~ao ~

N

I

OQ

lJ1

(1)

"'"

OQ

(f)

:::2:

0:: W fW

5

10

I

I

40

I

I

80

ILM. GRAIN SIZE (µ.m)

,.

I -

0

I

I !

I

20

PERCENT

10 C7

I

I

I

I

(c)

30 0,6

I

I

I

I

Fe203'

0,8

I

I I I I

I

I

FeO

1,0

>

,,,

!

I

1,2

I 0

I ~

J200X104

5

!

EMU/GM

10

10

5

10

MAKAOPUHI

LAVA

LAKE I

500

I

Drill Hole # II Mokoopuhi

600

I

III6

(11-9) (.

,.......·_·'""1(11-11) (11-10)

i

101-

.

ZONE \ 8 , OF HIGH I DRILL HOLE, OXIDATION . 10

N

~

12

....J

14

; o

r

No.ll

I

795-8651MC:I.O 10-13 MR-I.O

!

, ,

(11-12)

I

.

,

.

16

~ -

18 I

20

2 9

8

7

3

4

I 5

6

_frQ_ F~203 (b)

Figure Hg-Zl.

TOC-f02

in the cooling Makaopuhi hematite,

relationships

for in situ measurements

lava lake, Hawaii.

and the fayalite-magnetite-quartz

from Sato and Wright

(1966).

buffers,

respectively.

(b) Oxide data of samples

as a function

of the depth of collection,

from Haggerty

(1971).

Hg-92

of drill holes

MH and FMQ are the magnetite-

FeO:Fe 0 , 2 3

Data are

for drill hole #11

TOC and fOz'

Data are

7

FJOI:~

:f

4

i~[. I , I , SE:VA~E

0 Q 6

10

Z

20

30

I

40

I

•

]sELVAGE 50

60

70

80

50

60

70

80

I: 121 ANALYSES ~

(Figs. EG-llb,

trends demonstrate

early chromites

~1,1,~' ;

/

These

that

in these two basalts

from a liquid with

continuous

Al and thus before

crystallization

grew

build-up

of

of pla-

;:

gioclase.

2 ,

+

The Cr/Al substitutional

trend

for early chroillites in both rocks is entire-

++++++++-

ly different

from the trends

of other chro-

(5

mite generations

0

co

zr

it is quite similar trend reported

0

'" N

Apollo with 1. 20

2.40

3.60

4.80

AL

positive

and ulvospinel.

slope

This curvature

6.00

curved

for MgAlZ04-rich

14 samples

the second

However,

to the substitutional

(Haggerty,

chemical

substitutional

spinels

197Za).

cores at a Cr/Al ratio of 4:1 with

to Z:l and with

a sharp

was interpreted

as indicative

turn back to ratios higher

EG-15

of change

Spinels

trend display

trend starting

a

for

a steep

than 4:1.

in the activities

in

of

o

Figure EG-IZ.

15065.93 SPINELS CATIONS 264 ANALYSES

o

N

Cr-Al substitutional

trends in spinels

o

pigeonite

:

'",

~

e-

N

~

:

.~

+

3.00 RL

CR

3.00

2.00

4.00

5.00

RL

Figure EG-13. V-Al and V-Cr substitutional relationships in an olivine basalt. (a) (upper left) and (b) (upper right) substitutional trends of early spinels indicating increase in V and Al from core to rim. (c) (lower left) and (d) (lower right) substitutional trends for later chromite and ulvospinel. EG-17

pyroxene

(Laul and Schmitt,

pyroxene

entry as a crystallizing

provide

a good control

crystallization trends

sequence.

before

V-Al sympathetic for chromites pathetic

Figures

rocks.

(Figs. EG-13a,c,

crystallization

phase.

for the position

in two different

spinels

1973) and hence

Fig. EG-14a,c)

the second

substitutional

antipathetic

trends in the

the substitutional

trend of the early

is also well developed

and plagioclase

chemical

check for

and plagioclase

EG-13 and EG-14 document

trend and negative

with

an additional

of both pyroxene

The unique

pyroxene

it provides

V-Cr and V-Al substitutional

as evidenced

thus indicating

by the positive

V-Cr relationship.

The trends

trend and the late ulv~spinels

are anti-

both for V-Cr and V-Al. 15065.93 SPINELS CATIONS 106 ANALYSES

o

'"

15065,93 SPINELS CAT IONS 106 ANALYSES

g~

.

.... .'

id

iA'"'-f ""

~r

./

~

s

co

'C

2.40

7.20

4.80

9.60

12.00

1. 00

2.00

CR

3.00

Y.OO

5.00

AL 15065.93 SPINELS CATI ONS 158 ANALYSES

gr

15065.93 SPINELS CATIONS 158 ANALYSES

'" 0: x

'!

~

""

""

N

N

~ ~ co o

'" 'C

l,

+

~ 2.40

7.20

4.80

9.60

12.00

1.00

CR

2.00

~

3.00

Y.OO

5.00

Figure EG-14. V-Al and V-Cr substitutional relationships in a pigeonite basalt. (a) (upper left) and (b) (upper right) Substitutional trends of early spinels indicating increase in V and Al from core to rim. (c) (lower left) and (d) (lower right) Substitutional trends for later chromite and ulvDspinel.

EG-18

Fe-Mg substitutionaZ Several in Figures

trends

features

EG-15 and EG-16.

V-Cr, V-Al diagrams chromite

increasing

compositions

linear slopes

vertical

is demonstrated

cross the several

constructed various served

for compositions

generations

with similar

crystallized

for early spinels would

tutional

trends

proposed

ulvospinel

at different

for Mg from

generations

Furthermore,

slopes

connect spinels

of the

times and the relationship

then completely

disappear.

of the second zoning trend demonstrate

in the FFM ratio of the liquid after precipitation

are

slope lines for compo-

by Haggerty. Ti-content

does not

the Fe-Mg relationship in Figures EG-15 and EG-

evident.

Ti-contents

spinel

This attempt

Fe substitution

and chromian

In fact, these steep trends

in-

of the

would correlate

ratios.

core to rim for the later Ti-chromite

sitions with various

argued

if the Ti content

by Haggerty

increasing

Haggerty

is poor to totally

trend and even obscures

This conclusion

slope with

This trend very probably

FFM ratio.

TiOZ/(Tioz+crZ03+Alz03)

trends with sharply

All Tiemerge.

trend with negative

do emerge

Each slope proposed

in the zoned spinels.

in the Cr-Al,

initial FB1 ratios

Fe versus Mg relationship

the real substitutional

16, where

with various

from a liquid with decreasing

with similar

trends observed

for Fe from core to rim.

and that distinct

trends shown

in the Fe-Mg relationships.

a unique antipathetic

that the divalent

is considered.

reflect

generations

again display

growth

(197Za,b,c) coherent

All the substitutional

Mg substitutions

indicates

in the Fe-Mg substitutional

are also encountered

and ulvospinel

Early spinels

spinels

can be recognized

ob-

The Fe-Mg substi-

a continuous

of olivine

increase

(El Goresy et aZ. ,

1976).

Ti-(V+Cr+AZJ substitutions The Ti-(V+Cr+Al) in the normal-inverse tions. points

substitutional

ratio displays

solid solution

series

Non-stoichiometry from the

8

of the spinels

should

(Ti) to the 16 (V+Cr+A1)

from the 8:16 ratio was found and hence

either

V and Si in the spinel analyses the B site is deficient

occupancy

for divalent

tent

(~16%).

Spinel

trends

No evidence

(Fig. EG-17).

is responsible

or there is increase

ca-

of the data

of departure

is questionable Neglecting

for speculation

in the octahedral

to that site

cations.

The Luna 16 mare type basalts

variation

ratio.

in the B site

and trivalent

cause a departure

cation deficiency

(Nehru et aZ., 1974; El Goresy et aZ., 1976) include

the occupancy

of tetravalent

analysis

to Apollo

are characterized

by their high A1 0 conZ 3 in these rocks indicate similar compositional

lZ and Apollo

15 basalts

EG-19

(Haggerty,

197Z).

However,

15555.42 SPINELS CRTI ONS 146 RNRLYSES ~

0 0

::;

io 0>

0 0

~I 0 0

uJ LL

--

0 0

Figure EG-15. tutional

zoned spinels basalt.

~

Fe-Mg substi-

trends for various in an olivine

Note antipathetic

negative

trend for early

spinels

towards higher Mg

substitutions.

I

~~

Zoning trends

of later chromites and indicate

are steep

sharp Fe substi-

m

tution for Mg.

Substitutional

trends of zoned grains cross

0 0

several of the slope lines by

r-

Haggerty

1. 50

1. 00

.50

2.00

(1972a,b,c).

2.50

MG

15065.93 SPINELS CRTIONS 263 RNRLYSES

0 0 Lf)

Figure EG-16. tutional

zoned spinels basalt.

Fe-Mg substi-

0 0

trends for various

Antipathetic

tive trends

generations

0 0

nega-

for early spinels

is more pronounced Fig. EG-15.

(T1

in a pigeonite

than in

All chromite

uJ LL

--

0 0

ai

with various

FFM ratios emerge.

0 0

,-..:

.50

l. 00

l. 50 MG

EG-ZO

2.00

2.50

6

;.:: 2

6

8

12

10

Cr. AI. V Figure EG-17.

Plot of Ti+Si against Cr+Al+V

rake samples.

Open circles,

spinels;

cations for spinels from Apollo 15

filled circles,

chromite

and ulv~spinel

(from Nehru et aZ., 1974).

they are characterized by considerable solid solution towards FeA1204 and MgA1 0 (Fig. EG-18). Evidently, these spinels crystallized from a liquid 2 4 with high but continuously decreasing Al concentrations (Fig. EG-19) as documented

by the sharp decrease

comparison

in the Al/Cr ratio.

This may indicate

12 and 15 basalts,

anorthitic

co-precipitated

from the Luna 16 magmas

(Bence et

crystallization

of anorthitic

drastic

to Apollo

decrease

in the Al/Cr ratio.

types from Apollo Ilmenite (Haggerty, usually

lZ, Apollo

textures

would

the majority

concentrations

of the TiOZ-poor

probably

after

that in

and pyroxene

1972).

Continuous

the continuous from various

and basalt

15 and Luna 16 sites are shown in Table EG-l.

in basalts

in direct relationship

al.,

explain

Spinel compositions

of the three landing sites are quite similar

1971; El Goresy et aZ., 1971).

high geikielite

sequence

plagioclase

plagioclase

The concentration

of geikielite

is

with the total MgO content of the rock, e.g., in rocks with high MgO content. basalts

is usually

the late Cr-ulvDspinel.

EG-Zl

Ilmenite

in

late in the crystallization

Figure EG-18. (from Haggerty,

Compositions

of Luna 16 spinels

in the modified

spinel prism

1972b).

"r-------------------------,-----------~--------_, LUNA

16

SPINElS

'0

Figure

EG-19.

The dashed

.;

Atomic proportions

lines indicate

.',~

of Cr as a function

the zonal trends

EG-22

...

of Al for Luna 16 spinels.

(from Haggerty,

1972b).

Table

EG-l.

2 Si02 Ti02 Cr 0 2 3 A1203 VZ03 FeO MgO MnO CaO Total

6.74 42.20 11.10 0.97 36.60 1.89 0.40 0.05 99.50

25.80 16.30 4.37 0·62 51.30 1.70 0.36 0.05 99.80

Chemical

3 0.52 3.60 47.10 10.70 0.87 32.80 2.94 0.35 0.34 99.22

analyses

4 0.11 32.60 1.28 2.58 0.01 62.70 0.52 0.31 0.26 100.36

of spinels.

5

6

7

0.17 7.09 30.96 21.51

0.76 0.92 51.49 14.48

0.35 29.700 5.54 1.89

34.42 4.60 0.36 0.08 99.19

25.85 6.67 0.46 0.38 101.01

61.05 0.17 0.46 0.17 99.33

1: Titanian chromi.t:e,Apol-lo 12 (Taylor et al.1971, Table 5, p.865) 2: Chromian ul.ooepd.nel., Apo l.Lo 12 (Taylor et al., 1971, Table 5, p.865) 3: Magnesian-aluminian ahromite, Apollo 15 (Nehru et al.1974, Table 2, p. 1225) 4: Chromian ul.ooep inel., Apollo 15 (Nehru et al.1974, Table 2, p.1225) 5: Magnesian-aluminian ahromite, Luna 16 (Haggerty, 1972 b, Table I, p , 335) 6: Cr-riah aluminian-magnesian ahromite, Luna 16 (Haggerty, 1972 b, Table I, p. 334) 7: Chromian ulvospinel, Luna 16 (Haggerty, 1972 b, Table I, p. 335)

EG-23

Opaque

oxides

in TiOZ-rich

TiOZ-rich

basalts

Mare Tranquillitatis titanium mainly

basalts

confined

titanium

basalts

poikilitic

11 site and is restricted

secondary

titanian

by Mg-rich

ilmenite

poikilitic

il-

in the Apollo

chromian

interest

silicates

of armalcolite

encoun-

ulvDspinel,

are textural and opaque

chemistry

in the two different

present

re-

oxides

and opaque

major

rock

Smyth and Brett

basalts.

(1974).

Textural

11 TiOZ-rich

Preliminary

(1973) demonstrated

in terms of crystal

basalts

indicate Apollo

investigations

indicated

that the two armalcolite out the pos-

Their study, however,

that only the grey variety 17 plagioclase

in the Apollo

that armalcolite

morphology

ilmenite

is due to local variation

paragenetic

there are indeed

ilmenite

and coarse-grained

structure.ruling

polymorphs.

have equivalents

than to major

porphyritic

in medium-

usu-

showed

in MgO and CrZ0 contents as reported by El Goresy et al. 3 relationships of armalcolite-bearing assemblages in Apollo

is present.

basalts

Apollo

MgO and Cr 0 contents than the tan Z 3 for the difference in color (El Goresy

that the types are different differences

in olivine

in several

(a) a grey variety

shows higher

and this may be responsible

similar

types were reported

et al., 1974):

encountered

ilmenite

types are indistinguishable

ilmenite

armalcolite

1973; El Goresy

and (b) a tan variety

et al., 1973). sibility

porphyritic

in their Cr and Fe/Mg ratios.

that the grey armalcolite variety

Of special

Armalcolite

types:

The opaque oxides

armalcolite,

high-

sites

into two major

and (Z) olivine

and the coexisting

rocks.

for both landing

17 samples.

and rutile.

different

(Haggerty,

plagioclase

source

type was not encountered

ilmenite,

the earth.

relationships

ally mantled basalts;

basalts;

rocks as well as variations

Two optically 17 basalts

are:

armalcolite

types show differences

These high-

among the Taurus-Littrow

can be classified

to the Apollo

basalts

in the different

Armalaolite

ilmenite

chromite,

between

in the different oxides

of a similar

So far, the first basalt

tered in the studied

lationships

similarity

basalts

from

from 3.82 to 3.55 G.Y. and are

half of the side of the moon facing

and compositional

are suggestive

basalts.

11 and 17 flights

Site, respectively.

formed in the period

The TiOZ-rich

(1) plagioclase menite

during the Apollo

and the Taurus-Littrow

to the eastern

variations

1974).

were collected

were

Textural

(LSPET,

basalts

11 landing

in olivine

differences.

differences

poikilitic

EG-Z4

and plagioclase

in silicate

in the paragenetic

by Mg-rich basalts

do not

Pap ike et al. (1974) report

site.

porphyritic

According

mantled

ilmenite

poikilitic

crystallization

rather

to El Goresy et al. (1974) sequence

between

plagioclase

poikilitic

and olivine porphyritic

differences

are outlined

Plagioalase

1.

poikilitia

terized by the presence

basalts.

ilmenite basalts:

of two pyroxenes: as single

Kushiro,

and Cr-ulvDspinel

1974).

Ilmenite, lize.

Olivine

followed

exclusively

crystal

by tan armalcolite

plagioclase

Armalcolite

and paragenetic

overgrowths

with sectoral

on augite

by titanaugite

were the last minerals

as inclusions

in the titanaugite

ally, it is present

with ilmenite

in sealed grain boundaries

blocky

(Haggerty,

appearance

of titanaugite. the majority regardless observed,

Figure

1973).

The dominant

of armalcolite

crystal

In these rocks massive

of the pyroxenes

feature,

clusters ilmenite

crystallized.

of their grain size, were ilmenite although

Papike

(Fig. EG-ZO);

to crystal-

is the idiomorphic only in the cores

precipitation

started

In none of the studied reactions

and is

occasion-

with armalcolite

however,

occurring

to

and pigeonite.

in these rocks is only of the tan variety

present

morphology

(Hodges and

were among the first minerals

followed

and then cristobalite

occurring

This rock type is charac-

(a) titanaugite

zoning and (b) pigeonite

crystallize

These textural

below.

after

fragments,

rims around armalcolite

et al. (1974) report ilmenite reaction rims in sample

EG-ZO.

Cluster

of idiomorphic

tan armalcolite

clinopyroxene.

Apollo

17 plagioclase

poikilitic

field 400 microns.

EG-25

crystals

ilmenite

enclosed

basalt.

in a

Length

of

70035.

The above-described

cooling

rate.

crystallization

2.

sequence

ulvDspinel basalts,

of olivine

inclusions. were

plagioclase

These

(partially

These two minerals,

by grey armalcolite,

rocks are characterized

as a quench phase) with Cr-

as in the plagioclase

to crystallize

and then tridymite.

on the

et al. do not negate the

above.

phenocrysts

colite were not encountered. samples

by Papike

ilmenite basalts:

the first phases

then followed

path is not dependent

reported

described

Olivine porphyritic

by the presence

were

crystallizaticn

The few exceptions

then ilmenite

In coarse-grained The textures

poikilitic

et al., 1974).

(El Goresy

Both

and at last augite,

rocks olivine

and armal-

11 and Apollo 17

in both Apollo

are identical.

Two main features a.

b.

differentiate

Olivine

porphyritic

variety

regardless

In olivine

porphyritic

ilmenite

and pigeonite In olivine

tle vary from grain by continuous

ilmenite

by ilmenite to grain.

ilmenite

morphology. separately

The above-described

basalts, mehcanisms

basalts

The origin in detail

of the ilmenite grains

and the composite

grain still dis-

and shape of the mantling

between

sequence

rate to explain

light on numerous

basalts

in the olivine

and the inver-

porphyritic of different

these features.

in olivine porphyritic basalt

samples

that the ilmenite

are formed according

ilmenite

the two rock types, especially poikilitic

of the grain size of the rock, is suggestive

in reflected

man-

are surrounded

in a later section.

in plagioclase

1974; Papike et al., 1974) indicate grains

direct-

ilmenite

the major.ity of the armalcolite

the armalcolite

Origin of ilmenite rims around a~alcolite

malcolite

or not.

precipitated

poikilitic

Shape and width

(Fig. EG-Zl)

crystallization

other than cooling

Studies

ilmenite

after the major part of titanaugite

rims.

differences

of two pyroxenes

regardless

basalts,

by ilmenite

in plagioclase

Usually,

mantles

plays armalcolite

ted pyroxene-ilmenite

whereas

crystallized

will be discussed

the presence

ilmenite

only the grey armalcolite

contain

is mantled

precipitated.

porphyritic

grains are surrounded

basalts

if armalcolite

ly after armalcolite. basalts,

the two rock types.

ilmenite

basalts

(El Goresy

mantles

to one or a combination

around

et al. , grey ar-

of the following

processes: 1.

Reaction

between

the cooling

basaltic

liquid

and early crystallized

armalcolite FeTi 0 Z 5

+ FeO (from melt) ~ ZFeTi03 EG-26

Figure

EG-Zl.

and ilmenite

Several reaction

idiomorphic rims.

armalcolite

Olivine

crystals

prophyritic

displaying

ilmenite

bireflection

basalt.

Length

of

field ZOO microns.

Z.

Reaction

between

idealized

chromian

ulvDspinel

+

FeZTi0

4

3.

Reaction

between

and Lindsley

4.

Breakdown Simple

sample

1.

around

the major

armalcolite

11 sites

FeTi 0 Z 5

o

Fe

to the

(Lindsley

due to this reaction

~ 3FeTi0

3

and armalcolite

+ Feo ...5FeTi03 + "Ti305" to ilmenite

of ilmenite

around

of the above-described

in Apollo

The reaction

evidently

metallic

of armalcolite

overgrowth

The majority basalt

according

as suggested

by Harzman

(1973)

4FeTiZ05

5.

and armalcolite

reaction:

11 and Apollo

between

process

and rutile armalcolite.

processes

may be present

the cooling

basaltic

liquid

for the formation

basalts

enriched

EG-Z7

and armalcolite of ilmenite

of the Taurus-Littrow

et al., 1974; El Goresy et al., 1974). was apparently

in the same

17 material.

responsible

in the TiOZ-rich

(solid solution in armalcolite)

in TiO

Z

and Apollo

Ilmenite

in contrast

is

rims

formed

to primary

ilmenites mantles

precipitated

usually

cooling. where

directly

show numerous

Many

armalcolite

the basaltic

from the basaltic

rutile

grains

inclusions

show reactions

since the ilmenite

probably

that reaction

(1) is responsible

armalcolite.

In the fine-grained

no or little reaction

although

This feature

is indeed

for the formation vitrophyres

of pyroxene

and plagioclase

crystal

of the ilmenite

these armalcolites

(El Goresy

evidence rims around

grains

were not protected

quench

on

sides, namely

strong

a few armalcolite

This could be due to the very fast cooling

and the deposition

exsolved

only on certain

liquid had a free path to the armalcolite

et al., 1974; Papike et at., 1974).

silicates.

liquid,

which

display

by other

of the basaltic crystals

liquid

before

the re-

action started. Z.

Textures

strongly

lithic fragments

suggestive

and large basalts

ideal case pure ulvDspinel cording

Since ulvDspinel

in the Apollo solid solution

in addition

ulvospinel

will

Normally,

to ilmenite

change

17 landing

react with armalcolite

and ilmenite.

in a few

site.

In an

to form ilmenite

enrichment

reaction

as a result

stages

in MgA1 0 Z 4

ac-

due to this reaction

between

ulvDspinel

is also accompanied

(Fig. EG-Z3). with

spinel

by exsolu-

The chromite armalcolite

took place

9.3% in the original

were also probably

the chromian

of the reaction.

grain are still visible

deposited

the reaction

of ulvD-

will precip-

Thus,

to the degree

ulvDspinel

chromite

in the secondary

(20.5 wt. % AlZ03 versus

chromite

of this reaction.

the reaction

where

sense a member

titanian

according

of the original

boundaries

is in a broad

secondary

from the host ulvDspinel

fined to ulvDspinel Drastic

series,

the newly formed

In advanced

tion of ilmenite

17 basalts

its composition

the boundaries

(Fig. EG-ZZ) whereby

formed

(Z) were observed

to the equation

spinel-chromite itate

would

of reaction

from the Apollo

is con-

took place. due to this

ulvDspinel).

Ilmenites

rich in TiOZ' since rutile

ex-

solved from the ilmenites. 3.

Harzman

armalcolite

and Lindsley

heated

within

et al. (1974) report that

(1973) and Lindsley

its stability

field with metallic

iron yields

il-

4

menite + a different armalcolite in which part of Ti + is reduced to oxidize ss o Fe and the Ti3+ produced enters the armalcolite as Ti 0 component. El Goresy 3 5 et al. (1974) observed in many lithic fragments textures strongly suggestive of this reaction.

Several

armalcolite

with small iron globules ilmenite

mantle.

a significant

Electron

enrichment

grains mantled

at the boundary microprobe

between

analyses

of Ti compared

by ilmenite

the armalcolite

core and the

of these armalcolites

to the coexisting

EG-28

were observed

armalcolite

indicate in the

Figure EG-ZZ. ulvospinel ilmenite as marked boundary.

and ulvospinel.

(center,

Length

ian chromite

silicate

titanian

boundaries

in its lower part with

chromite

deposited

of ulvospinel·are

inclusions

between

still visible

above the ulvospinel-ilmenite

of field ZOO microns.

A very advanced

by secondary

left is ulvospinel

gray) which reacted

and secondary

Original

by small aligned

Figure EG-Z3. surrounded

Armalcolite

to form ilmenite

stage of reaction

ilmenite

with ilmenite

which

exsolved

exsolutions.

(dark gray) are located between

Length of field 150 microns.

EG-Z9

Z, gray at top is armalcolite rutile

(light gray).

Big patches ulvospinel

Gray at

of secondary

and armalcolite.

titan-

same lithic fragment. slightly

lower than those of coexisting

4.

Pure ferropseudobrookite

± 10·C

at and below 1140 also report enriched

spected material

+ ilmenite + rutile.

by ilmenite

11 TiOZ-rich

and before

overgrowths

in these

rocks.

composite

titanaugite.

An important

However,

of oxygen

between

basalts,

According

around

ilmenite

a common phenom-

criterion

precipitates

after

to this crystallization

pre-existing

armalcolite

to recognize

sequence

should be expected

this texture

is that the

grain does not show any resemblance

ppase relations

fugacity

(Usselman

tion of armalcolites sequence

to the armal-

is quite rare compared

to

of synthetic

TiOZ-rich

basalts

as a

(fo ) indicates that there is a direct relationship Z sequence and fOZ for basaltic liquids with the same

et al., 1975).

The observed

difference

and the reversal

of ilmenite

and pyroxene

in plagioclase

was found to be a function

Chemistry

were in-

in the 17

this reaction.

is, however,

this kind of overgrowth

the crystallization

composition

lization

the presence

1, Z, and 3.

Study of equilibrium function

and rutile satisfying

ilmenite

ilmenite-armalcolite

reactions

requires

11 and 17 basalts

basalts.

of ilmenite

colite morphology.

Apollo

to rutile + ilmenite

In olivine porphyritic

armalcolite

This breakdown

first to Mg-

and only very few grains of armalcolite

of armalcolite

enon in Apollo

poikilitic

of fO

(Usselman

Z

in the composiin the crystal-

and olivine porphyritic

basalts

et al., 1975) (Fig. EG-Z4).

of annalcolite

Haggerty

(1973) reports

compositionally

that tan armalcolite

indistinguishable

than 400 complete

analyses

tan and gray armalcolite culated

of a given Fe/Mg ratio decomposes

were found mantled

The breakdown

5.

armalcolites.

and rutile in almost 1:1 ratio.

for this reaction

4 as Ti +) are also

(Ti is calculated

(FeTi 0 ) decomposes to ilmenite and rutile Z 5 and Lindsley, 1973). Harzman and Lindsley

(Harzman

that armalcolite

armalcolite

of ilmenite

simple

The total cations

(El Goresy

on the basis of 5 oxygens never

Lind and Housley

analyzed

(1972) and Smyth

suggest

since the number

totalled

3.

(1973), armalcolite ordered whereby

.

~

are

abundances.

More

that both

of cations cal-

The total number

range from Z.9l to Z.97.

group Bbmm with the cations strongly

~

element

et al., 1974) strongly

are cation deficient

tions for all armalcolites

and gray armalcolite

in terms of major

of ca-

According

to

crystallizes in the space 4 Ti + cations occupy the

8f(M ) and Fe and Mg cations are randomly dLstributed among the 4C(Ml) Z Z sites. Following this model, the majority of the analyses revealed that Fe + and MgZ+ do not satisfy

the 4C site occupancy

EG-30

since they never

totalled

1,

-10.-----r----r-----,----.----,

74275 I

-II

/l' /II~ /

p,

\

Figure EG-Z4.

Melting

relations

of

-12

/

1

Apollo

17 sample

points

are those of O'Hara

phries

(1975) at their stated Gxygen

fugacities.

74275.

Triangular and Hum-

The iron-wUstite

(Fe-

FeO) curve is shown as reference. oul

Sp 0"

-15

Plin

_16L----L----'---..L----'-----' 1100

1300

1200

Temp.oC.

although

there is a full complement

two Ti cations per five oxygens 3 that Ti may be present as Ti + and 3 perhaps Cr as Cr2+. Wechsler et al. (1975) calculated 4-10% Ti2 +Ti05 compo3 nent for many lunar armalcolites, thus supporting the presence of Ti + rather (El Goresy

than cation

et al., 1974).

deficiency

of the armalcolite

The gray armalcolite and MgO contents Papike

of almost

Smyth suggested

variety

by relatively

(El Goresy et al., 1974).

than the tan variety

et al. (1975) indicate

structure.

is characterized

that many gray armalcolites

higher

CrZ03

However,

are zoned.

Papike

et al. report a decrease

in Cr 0 and increase in FeO content from the core Z 3 to the rim of an armalcolite grain. The compositional variation of a zoned crystal

was found to overlap

a major part of the separate

fields

assigned

for

tan and gray armalcolite. The Mg versus armalcolites tant features distribution (b) the Mg-Fe almost Mg.

are recognized:

substitutional with

(a) There

slope,

to olivine,

melt have a higher

The gray armalcolites,

not, show generally

relationship

a negative

in analogy

from the silicate later.

Cr cationic

distributions

EG-25 and EG-26,

of tan and gray armalcolites

coherent

Probably,

Fe and Mg Versus

are shown in Figures

is indeed a compositional with slight overlap

armalcolites Mg/Fe

Two imporbimodal

of the fields;

for the tan armalcolite

indicating

regardless

for tan and gray

respectively.

variety

that Fe is substituting which

crystallized

is for

earlier

ratio than those crystallized if they are mantled

higher Mg concentrations

EG-3l

by ilmenite

than tan armalcolite.

or

Compared

APOLLO 17

Mg~

samples

0.50

Q~l 0.40

i;

+

+ + ++ ++

l*:$.+ ~

.L

i

+

6+

+

+4t~:

/.«:1;:,

+

.L

+

+

'zfS!;;-A6

0.35;

70017,125 70215,159 72015,21 74242,19 74243,4 79155,63 70135,60 71135,29

."

:+

TS 6

6

~ 6

~"B

6 6 6 6 6

6

030 6

b Tan armalcolite t Grayarmalcolite 0.25+---,...----.---i.-...---r----.--l 0,35 0.45 0.40

Figure

EG-25.

Mg-Fe

cationic

substitutional

«s= 0,50

&

0.60Fe

0.55

relationship

(based on 5 oxygens)

for tan and gray armalcolite.

Mg 0.50

0.45

0.40

0.35

0.30

0,25

0.20

D55

.Q6O

Cr Figure

EG-26.

Mg-Cr

cationic

substitutional

armalcolites.

EG-3Z

relationship

for tan and gray

to tan armalcolite is not coherent. to enrichment described

the Mg-Fe substitutional

of armalcolite

above.

Electron

of Mg for armalcolite

the Mg-Cr

the scatter a positive

indicate

than for mantling

ilmenite

substitutional

relationship

partitioning

is also demonstrated

relationship

in the data points between

1, Z, 3, or 4

such strong preference

Mg and Cr.

this figure

The Mg-Cr

indicating

Armalcolites

et al., 1974).

(El Goresy in Figure

EG-Z6 which

for both armalcolite

for tan armalcolite,

is coherent,

behavior.

et al., 1974) as due

from reactions

analyses

bimodality

ship for gray armalcolite similar

microprobe

of the gray armalcolite

(El Goresy

in Mg resulting

rather

The compositional shows

relationship

This scatter was interpreted

types.

Despite

is suggestive

substitutional

that these two elements

in olivine

of

relation-

porphyritic

have

basalts

have

higher MgO and CrZ03 contents than tan armalcolites. The data presented here are also strongly suggestive of a partitioning of Mg and Cr between armalcolite and mantling menite

ilmenite rims around

(El Goresy

with

strong preference

gray armalcolite

of these elements

for armalcolite.

cores were also analyzed

et al., 1974), and the suggested

partitioning

with

is confirmed

in the plot

of MgO in armalcolite MgO in mantling ilmenite (Fig. EG-Z7). preference represent mantling after

15

The coherent

versus

CrZ0

3

positive

the actual partitioning

their formation

between

of the ilmenite

which may have caused

L.l.JIIAR ARMALOOUTES IlMENITES

This slope,

relationship

since the majority

ilmenite

slope of the data points

of both Mg and Cr for armalcolite.

ilmenite,

in mantling

indicates

however,

armalcolite mantles

an additional

a

may not and the

exsolved

rutile

redistribution

of

MgO Arm./MgO

11m.

and coexistilg

sa"""" 70Z15,159 72015,ZI

Figure

~tffl;~

3.0

742.43,4

EG-Z7.

versus

CrZ03 Arm./CrZ0 11m. re3 lationship for gray armalcolites

and mantling

1.07rrr-:;:"TT-r;;:rrT--r::r:rrrrT..--r~_j LO 1.5 Z.O 2.5 CrzO:l in Armalcol~.

3D

3.5

CrzO:l n I1merite EG-33

ilmenites.

Il-

the microprobe

both Cr and Mg between between

ilmenite

partitioning

Reflection

and rutile

and rutile.

shows higher

measurements

indicate

uous increase

from various

between

on numerous

armalcolite

above 600 nm compared

curves

Table EG-2.

O.IB 1B.43 5.47

O.OB 0.57

9B.B5

types from several

of tan armalcolite

in color.

Chemical

Armalcolite

analyses

3

0.30 71 .61 1.69 1.77 0.Z7 16.30 6.63 0.09 0.44 99.10

0.33 70.44 1.63 1.72

Thus,

than tan armalcolite. Apollo

17

show a contin-

compositions

in Table EG-Z.

Z SiOZ Ti02 CrZ03 AlZ03 VZ03 FeO MgO MnO CaO Total

to the

and ilmenite.

to a flat curve for gray armalcolite.

for the difference

rocks are displayed

of Cr and Mg

need to be similar

armalcolite

Mg and Cr concentrations

that the reflection

This is responsible

The partitioning

does not necessarily

of these two elements

gray armalcolite

basalts

ilmenite

of armalcolite.

5

6

74.30 Z.17 1.93

73.00 1.72 1.96

7Z.50 1.43 1.91

13.4 7.95 0.00

16.30 6.27 0.00

17.60 5.32 0.00

99.BO

99.Z0

98.80

4

O.OB

O.OB

74.13 Z.OO Z.10 0.10 14.0B

73.91 Z.OI 1.99 0.05 14.44 7.75 0.17 0.03 100.43

7.B6 0.14 0.04 100.53

1: 2: 3: 4: 5:

Tan armalcolite, Apollo 17 (El Goresy et al. 1974, Table 2, p. 641) Tan armalcolite, Apollo 17 (El Goresy et al. 1974, Table 2, p. 641) Gray armalcolite, Apollo 17 (El Goresy et al. 1974, Table 2, p.641) Gray armalcolite, Apollo 17 (El Goresy et al. 1974, Table 2, p. 641) Core of a gray armalcolite, Apollo 17 (Papike et aZ.1974, Table 6, p. 492) 6: Same gray armalcolite few microns away from the core, Apollo 17 (Papike et ol. 1974, Table 6, p. 492) 7: Same gray armalcolite just at the ilmenite rim, Apollo 17 (Papike et al. 1974, Table 6, p. 492)

Chromian ulvDspinel Chromian crystals

ulvospinel

occurs

but also as clusters

both rock types ulvospinel grained tered;

ilmenite instead,

the mesostasis. resemble

basalts

is probably

enclosed

The early

late-stage

crystallized

11 ulvospinel

types not only as idiomorphic in olivine

an early quench

this early crystallized

tiny discrete

the Apollo

in both basalt

of grains

ulvospinel

ulvospinel ulvospinels

In

In coarse-

was not encoun-

grains were observed in Apollo

since their composition EG-34

or pyroxene.

phase.

17 basalts

is intermediate

in

between

chromite

siderable

and ulvospinel.

solid solution

the composition the spinel tically

of ulvospinels

prism.

higher

The Apollo

towards MgAlZ04

and chromite

It is noteworthy

MnO and VZ0

exsolution

to mention

contents

3

17 ulvospinels,

(Fig. EG-28).

however,

EG-28 displays

lamellae

in ilmenite

that ulvospinel

n

AP Fe+silica 3. Incipient breakdown of ilmenite (only very few grains)

=

is direct evidence

from Apollo

Assemblages

and ru

due to the presence

curves

Fe or I =

in these two samples

more

in the univariant

are:

from their experimental

from the Taurus-Littrow

the first three reactions assemblages

Data

Fe (El Goresy et al., 1972; Haggerty,

reduction

reported

1-5.

Path of fugacity

Symbols

to the extensive

+

ilmenite

reactions

ferropseudobrookite,

basalts,

that these two rocks have undergone rocks,

=

fb

et al. (1972) conclude

that the presence

et al. (1972).

is uncertain.

to silica + Fe in addition

Taylor

7.6

80

for the reduction

taken from Sato et al. (1973) and Taylor below 830°C

8.4 n'/T'K

between

the assemblages

in 14053 and in the three Apollo

suggests

that the rocks of the two landing

tories.

Very probably,

during

a heating

950°C.

Fe

+

silica

of reduction

fugacity

explains

and ulvospinel

the Fe-ru-il

in order

and chromian ilmenite reduction occurs

ulvospinel

during

followed

Textures

an initial

in a mesostasis

assemblage

texture

fayalite

subsolidus

by rapid

cooling

breakdown

still displays

reduction

melt.

were placed close to 9500C

of both fayalite further

in this sample

accepted

of fayalite

assemblage,

of fayalite

always

to be the last

Reduction

of the mesostasis

the morphology

to

of ilmenite

14053 do not support

Fayalite

is widely

at the

thus preventing

of sample

process.

from the silicate

close to

of fayalite

assemblages

cooling,

which

have taken place after solidification breakdown

breakdown

Incipient

and mineralogy

assemblage,

to crystallize

of 830°C and/or

curve above 830°C and very probably

for the extensive

breakdown.

Fe.

that the opaque

fugacity

to account

+

his-

to reduction

to these temperatures

best the simultaneous

to ilmenite

to rutile + Fe may suggest below

in excess

upon heating

strongly

thermal

14053 and 14072 were subjected

event to a temperature

A mechanism

given oxygen

samples

17 basalts

sites had different

must

since the

grains

(El Goresy

et al., 1972). Apollo reduction ilmenite

17 samples

of chromian to Al-Ti

70017,

70035, and 70135 display

ulvospinel

chromite

to Al-Ti

chromite

+ rutile + Fe.

Accessory

does not show any sign of reduction

to Fe + silica.

vasive

and neglecting

reduction

1972; Taylor

of ilmenite

alone,

et al., 1972; Haggerty,

severely

semblages

shown in Table EG-4 suggest

place below

reduced.

1972a),

ples were

830°C,probably

to a foZ below

reduced

is only possible ilmenite

buffer

below

It is worthwhile

3

+

et al.,

that these samThe three as-

of these rocks took

These assemblages

830°C, where

the fayalite

to mention

and Nash

in the initial

An important blages

of the per-

(El Goresy

is unrealistic.

curve, but above

may have been

the I-Q-F curve.

buffer

aspect

are indeed

that the above-given

This

curve intersects

the

respectively,

phases

reaction

a model of opaque

to explain

of this model

temperatures

are subject

of Fe Ti0 and FeTi0 in the lunar 2 4 3 are by no means equal to 1.

(1975) discussed

et al. > 1974) of isochemical as Ti

fayalite

the Fe-ru-il

since the activities

and ilmenite,

Haselton

in these samples

curve.

to some correction ulvospinel

fayalite

On the basis

that the reduction

cooling.

subsolidus

one may conclude

Such a conclusion

during

extensive

+ ilmenite + Fe and of Mg

(formerly

proposed

the subsolidus

reactions

is the fact that the components

not in their standard

states.

EG-4l

by Lindsley

oxides with part of Ti present observed. of the assem-

Such a system would

display

deviation

from ideality.

This deviation

fOZ curve will not be a straight Ti-O with assemblages cate a compositional presented

that the

as a function

of temperature.

The mechanism

(1975) could explain the textures in the assemo blage armalcolite-ilmenite-rutile-Fe • However, it could hardly explain the breakdown features of chromian ulvospinel since Ti3+ was not detected in an ulvospinel-magnetite coexisting with metallic Feo between 1000 and l300°C ss (Simons, 1974). Nature

by Haselton

indicates

Experimental studies in the system Feo with metallic Fe (Simons, 1973) indeed indi-

coexisting variation

from stoichiometry

line.

of reducing

The causes

and Nash

agent in Apollo

for the reduction

since recovery

of the Apollo

17 basalts of lunar basalts

11 samples.

have been a subject

Several

mechanisms

of debate

were proposed:

(1)

internal

redistribution of valence of states of Fe, Ti, and Cr by assuming initial presence of Ti3+ and CrZ+ in the melt (Brett et al., 1971,197Z); (Z) vacuum pumping

of oxygen

197Z; Haggerty,

197Za);

(O'Hara et al., 1970; Biggar (3) sulfur

(4) alkali volatilization

carbon monoxide Vacuum because

pumping

of oxygen

of the preferential

basalts

since escape

of alkalis

of alkalis

or of redistribution

explain

as elements

as oxides

the precipitation

fugacities

cause oxidation,

The mechanisms

of the valence

of states

reduction

whereas

the oxidation

of sulfur

state

loss from lunar

of Fe, Ti, and Cr may well

iron from the lunar basaltic

for subsolidus

(Sato

state of the lunar

(NaZO) would not change

of metallic

to account

cause of reduction

the reduced

would

by carbon or

(Sato et al., 1973).

of Fe vapor at low oxygen

loss explain

(Brett, 1975);

(O'Hara et al.,

(5) reduction

than a few kilometers

(Sato et al., 1973).

of lunar basalts

they are unable

escape

as S

extrusion

could never be a plausible

Nor could alkali

volatilization

during

Ford et al., 197Z;

at a depth shallower

et al., 1973).

magmas

from lunar magmas

et al., 1971,197Z,

1970; Biggar

et al., 1971; Ford et al., Z-

loss from lunar magmas

reactions

liquids,

but

of iron-titanium

oxide assemblages. El Goresy solidification ship between

et al. (1975) propose

this gaseous

Many of the open cracks cleavage

that gaseous

of the lunar basalts.

in pyroxenes

activity

and subsolidus

penetrating

in several

silicate

Apollo

were found to be filled with metallic system

of veins

across

several

El Goresy et al. (1975) believe

activity

They found evidence

mineral

reactions

EG-42

17 samples

iron forms a network

grains along several

that these features

oxides.

as well as

15, and Apollo

The metallic

relation-

of the opaque

and oxide minerals

lZ, Apollo

iron.

took place after for genetic

exclude

hundred

microns.

the possibility

that iron liquid be explained

has been injected

in the crack system.

as due to deposition

these rocks after crystallization. filled cracks

display

textures

+ Feo. These reduction the ilmenite suggestive

grains.

cracks and reduction

cooling,

always

event is responsible

of opaque

(Fe(CO)5)

oxides.

was proposed

many of those complex

at different

temperatures

many of the open cracks, reduction

processes.

tinued during

compounds

to Feo+ CO. whereas

and permeated

those rocks after solidification

The reduction

process

drogen

an impact

during

in the plagioclase support

the reduction

the possibility

process

or ilmenite mechanism

of endogenic

processes

proposed

Upon

and break down iron would

would

account

fill

for the

may have con-

that these gases may the basalts

and formation

since no evidence

gaseous

of CO and carbonyle

metallic

from which

not the result

were found.

of iron in the

unstable

released

into

as strongly

these reactions.

It is also plausible

from the same magma chamber

is probably

become

and the reduction

down to ZOO°C.

these features

mixture

Upon breakdown,

carbon monoxide

to spinel + rutile

from the cracks radiating

for deposition

A gaseous

by such iron-

reduction

to have initiated

gaseous

The breakdown

cooling

have been released

originate

can

phase which permeated

grains penetrated

of subsolidus

El Goresy et al. (1975) consider

that a single

iron compounds

Ilmenite

typical

products

These observations

of iron from a gaseous

of a release of reheating

originated

of tension

cracks.

of solar wind hyor shock features

All these observations

do indeed

by Sato et al. (1973) and emphasize

activity

during

eruption

of lunar lavas.

REFERENCES Anderson, A. T., T. E. Bunch, E. N. Cameron, S. E. Haggerty, F. R. Boyd, L. W. Finger, O. B. James, K. Keil, M. Prinz, P. Ramdohr, and A. El Goresy (1970) Armalcolite: A new mineral from the Apollo 11 samples. Proc. Apollo 11 Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 1, 1, 55. Bence, A. E., W. Holzwarth, and J. J. Papike (197Z) Petrology of basaltic and monomineralic soil fragments from the Sea of Fertility. Earth Planet. Sci. Lett. 13, 299. Biggar, G. M., M. J. O'Hara, A. Peckett, and D. J. Humphries (1971) Lunar lavas and the achondrites: Petrogenesis of protohypersthene basalt in the mare lava lakes. Proc. Lunar Sci. Conf. 2nd, 617. Brett,

R. (1975) Reduction of mare basalts by sulfur loss The Lunar Science Institute, Houston.

(abstr.)

In, Lunar

Science VI, 89.

__~~~_' P. Butler, Jr., C. Meyer, Jr., A. M. Reid, H. Takeda, and R. J. Williams (1971) Apollo lZ igneous rocks 12004, lZ008, lZOOZ2: A mineralogical and petrological study. Proc. Lunar Sci. Conf. 2nd, 301. El Goresy, A., P. Ramdohr, and L. A. Taylor (1971) The opaque minerals in the lunar rocks from Oceanus Procellarum. Proc. Lunar Sci. Conf. 2nd, Z19.

EG-43

El Goresy, A., P. Ramdohr, and L. A. Taylor (197Z) Fra Mauro crystalline Mineralogy, geochemistry and subsolidus reaction of opaque minerals.

rocks:

Proc.

Lunar Sci. Conf. 3rd, 333. site:

~ Opaque

' and o. Medenbach (1973) Lunar samples from the Descartes mineralogy and geochemistry. Proc. Lunar Sci. Conf. 4th, 733.

, , and H.-J. Bernhardt (1974) Taurus-Littrow Ti02rich ba-s-a7l-t-s-:--70paque mineralogy and geochemistry. Proc. Lunar Sci. Conf.

5th, 627. and P. Ramdohr (1975) Subsolidus reduction of lunar opaque oxides: Textures, assemblages, geochemistry, and evidence for a late-stage endogenic gaseous mixture. Proc. Lunar Sci. Conf. 6th, 729. , M. Prinz, and P. Ramdohr (1976) Zoning in spinels as an indicator of ----t~h-e--crystallization histories of mare basalts. Proc. Lunar Sci. Conf. 7th, in press. Ford, C. E., G. M. Biggar, D. J. Humphries, G. Wilson, D. Dixon, and M. J. O'Hara (1972) Role of water in the evolution of lunar crust; an experimental study of sample 14310; an indication of lunar calc-alkaline volcanism. Proc. Lunar

Sci. Conf. 3rd, 207. Haggerty, S. E., F. R. Boyd, P. M. Bell, L. W. Finger, and W. B. Bryan (1970) Opaque minerals and olivine in lavas and breccias from Mare Tranquillitatis. Proc. Apollo 11 Lunar Sci. Conf., 513. and H.

o.

A. Meyer

(1970) Apollo

12:

Opaque

oxides.

Earth Planet.

Sc{:-Lett. 9, 379. (1972a) Apollo 14: Subsolidus reduction and compositional of spinels. Proc. Lunar Sci. Conf. 3pd, 305.

variations

(1972b) Luna 16: An opaque mineral study and systematic of compositional variations of spinels from Mare Fecunditatis. Sci. Lett. 13, 328.

examination

Earth Planet.

________ (197Zc) Solid solutions, subsolidus reduction and compositional characteristics of spinels in some Apollo 15 basalts. Meteoritics 7, 353. (1973a) Apollo 17: Armalcolite paragenesis of chromian-ulvospinel and chromian-picroilmenite Am. Geophys. U.J 54, 593.

and subsolidus reduction (abstr.). E@S (Trans.

(1973b) Armalcolite and genetically associated samples. Proc. Lunar Sci. Conf. 4th, 777.

minerals

in the lunar

(1973c) Luna 20: Mineral chemistry of spinel, pleonast, chromite, ulvospinel, ilmenite, and rutile. Geochim. Cosmochim. Acta 37, 857. Harzman, M. J. and D. H. Lindsley (1973) The armalcolite join (FeTi20s-MgTi20S) with and without excess Feo: Indirect evidence of Ti3~ on the moon (abstr). Ann. Meeting Geol. Soc. Am. 5, 593. Haselton, J. D. and W. P. Nash (1975) Observations on titanium in luna oxides and silicates (abstr.). In, Lunar Science VI, 343. The Lunar Science Institute, Houston. Hodges, F. N. and I. Kushiro (1974) Apollo 17 petrology and experimental determination of differentiation sequences in model moon compositions. Proc. Lunar Sci. Conf. 5th, 1, 505. Kesson, S. E. (1975) Mare basalts: Melting experiments pretations. Proc. Lunar Sci. Conf. 6th, 921.

EG-44

and petrogenetic

inter-

Knorring, O. V. and K. G. Cox (1961) Kennedyite, brookite series. Mineral. Mag. 32, 67Z.

a new mineral

of the pseudo-

Kushiro, I., Y. Nakamura, and S. Akimoto (1970) Crystallization of Cr-Ti spinel solid solutions in an Apollo lZ rock, and source rock of magmas of Apollo 12 rocks (abstr,}. Am. Geophys. U. Ann. Meeting, 64. Laul, J. C. and R. A. Schmitt (1973) Chemical composition 17 samples. Proc. Lunar Sci. Conf. 4th, 1349.

of Apollo

15, 16, and

Lindsley, D. H., S. E. Kesson, M. J. Hartzman, and M. K. Cushman (1974) The stability of armalcolite: Experimental studies in the system MgO-Fe-Ti-O. Proc. Lunar Sci. Conf. 5th, 1, 521. LSPET

(Lunar Sample

Preliminary

Examination Team) (1974) Preliminary L. B. Johnson Space Center.

Examination

of Lunar Samples, pp. 7.1-7.46.

Mao, H. K., A. El Goresy, and P. M. Bell (1974) Evidence of extensive chemical reduction in lunar regolith samples from the Apollo 11 site. Proc. Lunar

Sci. Conf. 5th, 673. Marvin, U. (1975) The perplexing behavior samples. Meteoritics 10, 452.

of Niobium

in meteorites

Muan, A., J. Hauck, and T. Lofall (1972) Equilibrium studies lunar rocks. Proc. Lunar Sci. Conf. 3rd, 1, 185.

and lunar

with a bearing

Nehru, C. E., M. Prinz, E. Dowty, and K. Keil (1974) Spinel-group ilmenite in Apollo 15 rake samples. Am. Mineral. 59, lZZO.

minerals

on and

O'Hara, J. M., G. M. Biggar, S. W. Richardson, and C. E. Ford (1970) The nature of seas, mascons, and the lunar interior in the light of experimental studies. Proc. Apollo 11 Lunar Sci. Conf., 695. ________ . and D. J. Humphries (1975) Armalcolite crystallization, phenocryst assemblages, eruption conditions and origin of eleven high titanium basalts from Taurus-Littrow (abstr.). In, Lunar Science VI, p. 616-618. The Lunar Science Institute, Houston. Papike, J. J., A. E. Bence, and D. H. Lindsley (1974) Mare basalts from the Taurus-Littrow region of the moon. Proc. Lunar Sci. Conf. 5th, 1, 471. Prinz, M., E. Dowty, K. Keil, and T. E. Bunch (1973a) Mineralogy, petrology and chemistry of lithic fragments from Luna 20 fines: Origin of the cumulative ANT suite and its relationship to high-alumina and mare basalts. Geochim. Cosmochim. Acta 37, 979. thosite

in Apollo

__~ ' and 16 samples. Science

Ramdohr, P. and A. El Goresy from mare Tranquillitatis.

(1973b) Spinel 179, 74.

(1970) Opaque minerals Science 167, 615.

troctolite

and anor-

in the lunar rocks and dust

Sato, M., N. L. Hickling, and J. E. McLane (1973) Oxygen Apollo 12, 14 and 15 lunar samples and reduced states

fugacity values of of lunar magmas.

Proc; Lunar Sci. Conf. 4th, 1061. Simons, B. (1974) Zusammensetzung und Phasenbreiten der Fe-Ti-Oxyde in Gleichgewicht mit metallischem Eisen. Diplomarbeit, Technische Hochschule,

Aachen, 104. Smyth, J. R. and P. R. Brett (1973) The crystal structure of armalcolites Apollo 17 (abstr.) Ann. Meeting Geol. Soc. Am. 5 (7), 814.

~anet.

(1974) The crystal chemistry Sci. Lett. 24, 262.

of armalcolites

EG-45

from Apollo

17.

from

Earth

Taylor, L. A., G. Kullerud, tural features of Apollo

and W. B. Bryan (1971) Opaque mineralogy and tex12 samples and a comparison with Apollo 11 rocks.

Proc. Lunar Sci. Conf. 2nd, 855. __~~ __ ' R. J. Williams, and R. H. McCallister (1972) Stability relations of ilmenite and ulvospinel in the Fe-Ti-O system and applications of these data to lunar mineral assemblages. Earth Planet. Sci. Lett. 16, 282. Usselman, T. M. (1975) Ilmenite chemistry in mare basalts, an experimental study. Origin of mare basalts and their implications for lunar evaluation (abstr.). In, Lunar Science, 164. The Lunar Science Institute, Houston. ________ and G. E. Lofgren (1976) Phase relations of high-titanium rare basalts as a function of oxygerr fugacity (abstr.). Lunar Science VII, 888. The Lunar Science Institute, Houston. Wechsler, B. A., C. T. Prewitt, and J. J. Papike (1975) Structure of lunar and synthetic armalcolite (abstr.) In, Lunar Science The Lunar Science Institute, Houston.

and chemistry

VI, 860.

Williams, R. J. (1971) Reaction constants in the system Fe-MgO-Si02-02 at 1 atmosphere between 900°C and l300°C: Experimental results. Am. J. Sci.

270, 334.

EG-46

OPAQUE

OXIDE MINERALS

in METEORITES

Ahmed EZ Goresy

Chapter 6 INTRODUCTION

Records

of stones

Chinese

and ancient

Alsace,

France,

weighing

falling

from the sky can be traced back to classical

Greek or Latin

literature.

is the oldest preserved

127 kg fell on November

corded by chroniclers

of the town in a detailed

26, 1803 that the majority

the extraterrestrial

origin

and carefully

due to the fact that these objects

e.g., (1) processes

processes,

(2) processes

those resulting

it is widely

accepted

The wide variation meteorites

petrologists (1920).

comprise

contain

spherical objects

as chondrites are chemically the same type

silicate

actually

or oxide objects in achondrites. no chondrules

and mineralogically (Mason,

1962).

similar

The chondrites

EG-47

objects

belt.

from which

in meteorite

of meteorites

among

is based on that of Prior Chondrites

since silicate The distinction

is straightforward:

contain

creating

come from the asteroid

is shown in Table EG-S.

are absent

objects

by the variations

as stony meteorites

of stony meteorites

of and (3)

to the earth;

of the primary

(1962) which

the major part of these meteorites.

categories

spherical

together

is mainly

of solar system

interplanetary

used classification

of Mason

This classification

The increasing

is known about the exact source of meteorites,

is well demonstrated

is the scheme

falls.

prior to formation

analogous

that they probably

The most commonly

could be grouped

bodies,

stones

cofmnunity all ove r the

the wide variety

in the compositions

were derived

compositions.

document

accepted

demonstration

in the last 100 years,

events between

Little

finally

How-

fell on

that the L'Aigle

meteorite

especially

in the solar nebula

from collisional

shock and fragmentation. although

community

Franyaise

recorded

in planet-like

description.

France which

due to the convincing

of the Academie

in the study of meteorites,

planets;

A stony meteorite

illustrated

of the scientific

i~

and the event was re-

Ever since that time the scientific

world has continuously interest

fall.

shower of L'Aigle,

of meteorites,

by J. B. Biot to the members fell from the sky.

meteorite

16, 1492 in Ensisheim

ever, it was only after the meteorite April

The stone of Ensisheim

called

between

"chondritic" chondrules

However,

and achondrites

and oxide minerals

whereas

a few stones

but are so classed

the two

meteorites these

considered

because

to the chondrule-bearing are the most abundant

they

stones of all

of

Table EG-5.

Classification

Class

of meteorites.~

Subclass

I. Chondrites

A. B. C. D.

II.Achondrites

A. Calcium-poor achondrites 1. Enstatite achondrites (aubrites) 2. Hypersthene achondrites (diogenites) 3. Olivine achondrites (chassignites) 4. Olivine-pigeonite achondrites (ureilites) B. Calcium rich achondrites 1. Augite achondrites (angrites) 2. Diopside-olivine achondrites (nakhlites) 3. Pyroxene-plagioclase achondrites a) Eucrites b) Howardites

III. Stony irons

A. B. C. D.

IV. Irons

A. Hexahedrites B. Octahedrites 1. Coarsest octahedrites 2. Coarse octahedrite 3. Medium octahedrites 4. Fine octahedrites 5. Finest octahedrite C. Nickel-rich ataxites

"From

Mason,

meteorites.

of meteorites;

of about 5.5% Ni; it crystallizes

(b) Gamma iron, or taenite is a nickel-iron

lattice.

sition ranging cabic lattice.

rich ataxites

Most iron meteorites

with cleavage of kamacite

of iron meteorites

are mixtures

surface.

parallel

Hexahedrites

and taenite bands are parallel

EG-48

and Ni-

and taenite

In octahedrites

to octahedral

compo-

and taenite.

octahedrites,

consist of large crystals

to the faces of a cube.

cubic

in a face-centered

of both kamacite

of kamacite

of

alloy with

alloy of variable

into hexahedrites,

is mainly based on the configuration

etched

it consists

in a body-centered

from about 27 to about 65% Ni; it crystallizes

The classification

polished

in the majority

(a) Alpha iron or kamacite is an iron-nickel

alloys:

composition

Olivine stony irons (pallasites) Bronzite-trydimite stony irons (siderophyres) Bronzite-olivine stony irons (lodranites) Pyroxene-plagioclase stony irons (mesosiderites)

1967

A metal phase occurs

two nickel-iron constant

1962,

Enstatite chondrites Olivine-bronzite chondrites Olivine-hypersthene chondrites Carbonaceous chondrites

planes.

in a

of kamacite the orientation This structure

is also known as "Widmanstatten

pattern"

after its discoveror,

Widmanstatten.

The texture of the pattern

lationship

the Ni content of the meteorite.

with

the finer the octahedral creases,

structure.

the bands of kamacite

tremely narrow Meteorites

and discontinuous,

In stony meteorites, accessory

minerals

As the nickel content

chondrites

the abundance,

(e.g., carbonaceous

assemblages,

chondrites)

mass of the same meteorite.

and textures

In iron meteorites,

opaque oxides

occur intergrown

with troilite

(stoichiometric

Members magnetite) rutile brookite

groundmass.

In

of opaque oxide minerals

or in Ca, Al-rich

inclusions

from those in the groud-

opaque oxides are extremely

in which they are encountered.

with silicates

in silicate-rich

FeS) and graphite which

The

inclusions

or

also occurs as macrp-

in the NiFe alloy groundmass.

of the spinel group

are the most abundant

constitute

disappears.

opaque oxides occur as

can vary drastically

to the mass of meteorites

pattern ataxites.

in the silicate

chondrites)

rare compared

scopic inclusions

inex-

and achondrites,

(e.g., in many ordinary

in chondrules

of octahedrites

as nickel-rich

evenly dispersed

re-

the Ni content

At 12-14% Ni, they become

and the Widmanstatten

chondrites

usually

The higher

become narrower.

of this type are classified

Baron Alois von

is in direct but inverse

(chromite,

only a small fraction

achondrites

and to a lesser extent

of the oxides.

series have never been encountered

drites and enstatite

spinel,

opaque oxides in meteorites.

in meteorites.

appear to be barren

Ilmenite

Members

and

of the pseudo-

Enstatite

chon-

of opaque oxides.

MINERALOGY

Spinel group minerals Chromite meteorites.

is by far the most abundant Chromite

stony meteorites accessory However,

(Jedwab,

of carbonaceous 1971; Ramdohr,

titanomagnetite

end member

quantities

and achondrites) palasites,

and is the most abundant

and iron meteorites

chondrites

1973).

is the dominant

of the spinel series in

in more than 90% of all

is especially

In a few calcium-rich spinel

(El Goresy,

chondrites. enriched

oxide 1965).

Instead,

the

in magnetite

achondrites

(Nakhlites)

(Bunch and Reid, 1975; Boctor et aZ.,

Spinel

chondrules major

(chondrites

in mesosiderites,

in various

it is rare or almost absent in carbonaceous

groundmass

1976).

occurs

(MgA1204) occurs as a minor component especially in a few in ordinary chondrites (Ramdohr, 1973); however, the mineral is a

constituent

(Sztrokay,

of Ca and Al-rich

1960; Christophe

inclusions

Michel-Levy,

in some carbonaceous

1969; Marvin

EG-49

et al., 1970).

chondrites Many of

these Ca- and Al-rich

inclusions

perovskite

accepted

mordial

are widely

solar nebula

(Grossman,

Based on textures lowing various (2) clusters chromite;

types of chromites

of chromite

in melilite,

spinel,

pyroxene,

condensates

and

from the pri-

1975).

and assemblages,

(5) chromite

The number

enriched

as high-temperature

aggregates;

chondrules,

of these groups would

Ramdohr

in ordinary

(1967,1973)

(3) pseudomorphous and (6) myrmekitic

increase

recognized

chondrites:

markedly,

the fol-

(1) coarse chroroite; chromite;

(4) exsolution

(or symplectitic)

chromite.

if subtle morphological

de-

tails were taken into account. 1.

Coarse

of ordinary present

chromite:

chondrites,

as coarse euhedral

matrix.

Coarse

troilite

chromite

or metallic

usually

anhedral.

olivine

and pyroxene

of 73 chondrites varies

between

the chromite groups).

microns

sequence.

type

(L

along

to silicates

chromite

content

relationship

=

integration of chondrites

was found between

low iron, or H (111) planes

but not rare in chromites

are

is later than

Planimetric

that the chromite

of ilmenite

in the silicate

only if in contact with

Its boundaries

No apparent

and the chondrite

in chondrites,

interlocked features

(1973), coarse

in the crystallization

lamellae

majority It is

=

high iron

of coarse

chromite

in mesosiderites

or

(Fig. EG-3l).

EG-31.

in pyroxene

1973).

to Ramdohr

0.01 and 0.61 wt %.

content

grains

idiomorphic

iron (Ramdohr,

According

Exsolution

pallasites

to subhedral

exhibits

type occurs in the overwhelming

stony irons, and iron meteorites.

(Keil, 1962) indicates

is a rare feature

Figure

This chromite achondrites,

Mount Padbury

with

rutile

(from Ramdohr,

(stony iron), Australia.

exsolution

lamellae

unpublished).

EG-50

A grain of coarse

(white); length

of photograph

chromite 150

2.

Clusters

ordinary morphic

of medium-

usually

to fine-grained

embedded

in plagioclase

type is also later in the crystallization

to

idio(Fig.

sequence

than

and pyroxene.

Figure EG-32.

Nardoo chromite,

(from Ramdohr,

3.

Pseudomorphous

of albitic

volume.

plagioclase

chromite:

New South Wales.

length of photograph

oriented

content

occurs

Cluster

350 microns

ureyite

mite to albite.

(kosmochlor), Yoder

in which this type occurs and sometimes

to be restricted

clinopyroxene

(less than 10 microns

(Ramdohr,

Ramdohr

stony irons,

consists

chiefly (Ramdohr,

may be as high as 40% by

laths of a former unknown 1973).

(1976) suggested

in diameter)

mineral

which broke

The texture

is indeed

the precursor

is very

NaCrSi 0 , to account for the 1:3 ratio of chro2 6 (1971) report that at 700°C and 2 kb albite

and Kullerud

well as albite + eskolaite

ever, a mixture

minor

of such chondrules

(Fig. EG-33)

for breakdown.

type appears

It is absent in achondrites,

as fine grains

in fan-shaped

down to albite + chromite characteristic

This chromite

chondrites.

and chromite,

The chromite

+ chromite,as

chondrite),

is kamacite;

The assemblage

This chromite

frequently

white

in ordinary

and iron meteorites.

1967,1973).

(olivine-bronzite

1973).

to chondrules

ditions

This type seems to be restricted

consists

grains of chromite

This chromite

of aggregate

probably

aggregates:

The cluster

to subhedral

EG-32). olivine

of chromite

chondrites.

of kosmochlor

to form an albitic

+ anorthite

plagioclase

(Cr20 ), were found to be stable. How3 + enstatite reacted at the same con-

+ eskolaite + chromite + clinopyroxene. EG-5l

Figure EG-33.

Bachmut

(olivine-hypersthene

with pseudomorphous

chromite

(dark gray) matrix;

length of photograph

Yoder

and Kullerud

product

(1971) thus propose

or metastable

bronzite

quench product

and possibly

radiating

Ukraine.

A chondrule

pseudomorphs

in albite

1.2 mm (from Ramdohr,

that an explanation

+ albite (+ minor clinopyroxene)

chromite

chondrite),

in lath-shaped

1967).

for the assemblage

could be that kosmochlor

that reacts with anorthite

olivine to form albitic

plagioclase,

is a stable

as well as

chromite,

and a

clinopyroxene. 4.

Exsolution

chondrules pyroxene

and plagioclase.

rich silicate 5.

to account

Chromite

is spectacular chondrules. The chondrules clase.

chromite:

as fine-grained

Ramdohr

Though uncommon

This type of chondrule consist

(Ramdohr,

usually

chondrules

1967).

exsolution

in chondrites,

origin

between

clino-

from a chromium-

the chromite

and the chemical

was first discovered

of a two-phase

by Ramdohr

assemblage:

chromite

type

variation

of

(1967). and plagio-

ratio can vary drastically, and in some cases are encountered,

The chromite

grain size, compactness,

shell of the chondrule

in clinopyroxene-rich

at the boundaries

(1973) proposes

with regard to its possible

The chromite:plagioclase

of different

of chromite

for this assemblage.

chondrules:

almost pure chromite orites

This type is encountered

clusters

is usually

occurs

e.g., Loot and Burdette mete-

in concentric

and plagioclase

surrounded

EG-52

alternating

content.

by a thin feldspar

layers

The outermost layer of fairly

uniform

t~ickness.

overgrown

bv FeNi alloy and troilite.

an atoll-like

feature with a plagioclase

Harrisonville

chondrite.

terized

groundmass grains

which,

throughout

sealed by an almost from the meteorite

Figure

EG-34.

groundmass

common

with

of chromite

constituents,

of the chromite

occurs

rapid cooling.

These

ting a genetic dominance composition

usually

is charac-

in a plagioclase chromite

The chondru1e

is

in turn is separated

layer of plagioclase

(Fig. EG-34).

chondrite),

Hale County,

crystals,

fine-grained

chromite

and chromite

rim; length

exceeds

50% by volume

EG-35.

Although

clinopyroxene as skeletal chondrules

link between

of chromite

which

exhibit

shell, e.p. ,

of photograph

450

1967).

type is shown in Figure

the major

chondrule

small anhedral

olivine-bronzite

idiomorphic

intergrowth,

(from Ramdohr,

The amount

by a continuous

chromite

crystals

(Fig. EG-34).

shell of chromite,

(polymict

chondrule

chromite-plagioclase microns

chromite

the plagioclase

continuous

Plainview

Chromite

type of chromite

of large idiomorphic

in turn, is loaded with numerous

dispersed

Texas.

A very common

by the presence

Some chondrules

core and a uniform

crystals,

also contain

the two chromite

and plagioclase

of the liquid

occurs

from which

of the chondrule.

chromite

in minor

amounts.

indicative

EG-53

due to

chormite,

(Fig. EG-35).

in these chondrules the chondrules

are

A major part

of quenching

pseudomorphous types

A less

and plagioclase

document

were derived.

indica-

The prethe unusual

Figure

EG-35.

chromite length

Mangwendi

chondrule

(polymict

with skeletal

of photograph

450 microns

Myrmekitic

(symplectitic)

6.

to mesosiderites and chromite observed

(Fig. EG-36). 1973).

of olivine

Magnetite carbonaceous

gestive

spherules

Usually, (Ramdohr,

electron

as reaction

However,

of eutectic

microscope

of numerous

rims around metallic

the main mass is present

in the groundmass (Jedwab,

of stacking

(Fig. EG-37). 1971) indicates platelets

components

as

A detailed that these

or spirals

This interpretation

are one of the latest

occasionally

occur in Ca-rich

1975; Boctor et al., 1976); they usually (111) of the magnetite

nakhlites

are indicative

olivine

has been

would

to condense

sugindi-

directly

solar nebula.

Titanomagnetites

of the magnetite

of major

nor pyroxene

oxide in the groundmass

from the vapor phase.

cate that these magnetites from the cooling

1973).

are in fact composed

of condensation

consists

plagioclase

it occurs

sizes dispersed

study with the scanning magnetite

This type seems to be restricted

and assemblage

is by far the major opaque chondrites.

of various

A

chromite;

1973).

The assemblage

So far, neither

Rhodesia.

and pseudomorphous

and chromite.

FeNi alloy or troilite spherules

chromite:

Texture

chondrite),

of chromite

(from Ramdohr,

and iron meteorites.

(Ramdohr,

intergrowth

olivine-hypersthene

crystals

host.

and many

exhibit

and very fine lamellae These features

terrestrial

igneous

of ulvospinel

document rocks

achondrites

ilmenite

(Bunch and Reid,

lamellae

parallel

parallel

the close similarity

(Boctor et aZ., 1976).

to

to (100) between

Figure

EG-36.

tergrowth Ramdohr,

Vaca Muerta

between

chromite

unpublished)

....

.

•

•..... .I.. '

.,~.

~.'

Chile.

Myrmekitic

length of photograph

(symplectic) 800 microns

in(from

•

~ .. \"

.. .'

.

(mesosiderite), and olivine;

.'

...

..,.

...... .~.'

.

'"

~

Wi

.

M

....

.. .'. ......

• {~~,'~

_",.

....

'

.,

.'

.'

11·:'11\'" .. , •

,:~~.

. , .~.

.~ . .

...,,'

.

.

..

'

. r-

.,.. ... ., .~ til· ',\. "",

.L' • •. :--&..... .

'. .~

'

,'.' a '.';'

,

.......

. • r. "

_..

t •..

Figure EG-37. carbonaceous Ramdohr,

Esebi

(carbonaceous

and silicate

chondrites),

groundmass;

Zaire.

• AI· , .~ .

Magnetite

length of photograph

unpublished).

EG-55

spherules

300 microns

(from

in

Ilmenite-geikielite-pyrophanite Members

series

of this series were reported

and iron meteorites

(Ramdohr,

1965; Bunch and Keil,

1971).

1963,1973;

along with ulvospinel

in very few iron meteorites chromite. ilmenite

In contrast

1973).

titanohematite

Figure

presumably (Ramdohr,

EG-38.

morphic

Rutile

with shock-induced

lamellae

Only

in coarse

rocks, meteoritic

lamellae

are very rare (Ramdohr,

were found to exhibit collisions

chondrite),

twin lamellae,

(from Ramdohr,

in space

white

higher

(achondritic,

stony irons)

The FeD content

geikie1ite (MgTi0 ) 3 in chondri tic ilmenites

i1menites

(Snetsinger

of ilmenite

Antarctica. is troilite;

Xenolength

1973).

of the components

is usually

chondrites occurs as

e.g., Vacu Muerta

from meteorites

(olivine-hypersthene

Keil,

1971).

and metamorphic

formed by shock due to meteorite

600 microns

The amount in ilmenite

usually

as exsolution

lamellae.

reported

1969; El Goresy,

in carbonaceous

1973).

Adlie Land

ilmenite

of photograph

and Keil,

stony irons,

ss (Boctor et al., 1976).

in some mesosiderites,

Many of the ilmenites

(Fig. EG-38)

igneous

chondrites,

ilmenite

in titanomagnetite

is it also present

to be frequent

twin lamellae

In nakhlites,

to terrestrial

never displays

but reported

Snetsinger

They seem to be absent

and are very rare in achondrites. lamellae

from ordinary

(MnTi0 ) 3

than in nonchondritic

and Kei1,

in ordinary

EG-S6

and pyrophanite

1969; Bunch and

chondrites

tends to

increase

with

decrease

(Snetsinger

increasing

FeO/(FeO+MgO)

in coexisting

olivine,

but MgO tends to

and Keil, 1969).

Rutile Rutile be absent mineral

is quite rare in ordinary

in carbonaceous

in the majority

and many pallasites. iron meteorites

and achondrites

However,

it is a frequent

of mesosiderites

(Ramdohr,

is most

1965,1971).

abundant

Busek and Keil

in the Farmington

and stony irons the mineral

has a colorless

flected

internal

light with

chondrite

frequent

and Vaca Muerta

lamellae

in chromite

were

Ramdohr

(1964) suggested

aFe'was

probably

El Goresy

are characterized

also reported

(Ramdohr,

occurring

that rutile

with a greenish

by a relatively

that among

In chondrites

appearance

in re-

in ilmenite

mesosiderites

(Ram-

rutile

1965; Busek and Keil,

in the assemblage

1966).

rutile-ilmenite-

reaction

present

in iron meteorites

color in reflected

high Cr 0 2 3

OXIDE ASSEMBLAGES

1964) in

Both in the Farmington

in several

due to the subsolidus

reports

properties

(1966) report

it occurs as lamellae

However,

that rutile

formed

(1965,1971)

lous optical

1966).

and Klein, inclusions

chondrite.

to pale blueish

reflections.

mesosiderite

dohr, 1965; Busek and Keil,

and seems to accessory

1965; Marvin

The mir.era1 is also not rare in silicate

(El Goresy,

chondrites,rutile

chondrites

chondrites.

light.

(1.23%) and Nb0

(2.93%)

2

IN VARIOUS

METEORITE

has anomaThese rutiles contents.

GROUPS

Chondrites The classification

of chondritic

Table EG-5 is based mainly The abundance

of Fe, Mg, Si, and 0 altogether

may total 90% in the majority in contrast FeNi alloy. stones, These

to terrestrial Prior

the richer

of view, Prior's

will

control phase.

are known as Prior's

of meteorites

In reduced

contain

e.g"

EG-57

metallic

point

under which mete-

at a given

the coexisting

enstatite

silicates."

From the petrological

fugacity

before,

in chondritic

in Fe are the magnesium

rules.

oxygen

of iron between

chondrites,

As mentioned

of Ni-Fe

of the f02 conditions

the prevailing

the partitioning

is very high and

1974).

that "the less the amount

given in

of the meteorites.

in chondrites

it is in Ni, and the richer

since

in the four subclasses composition

(Wasson,

rocks the majority

rules are expressions

were formed,

metal

of chondrites

(1916) noted

two relationships

orites

meteorites

on the mineralogical

temperature

silicates

chondrites,

most

and the of the

iron is present spondingly dized;

in the metal phase,

low.

With increasing

the iron content

decreases

while

increases. ordinary

Prior's

the fractionation

chondrites olivines

(but not for enstatite

chondrites

Urey and Craig one having

as qualitative

or carbonaceous

could not be understood

(1953) demonstrated

and rhombic,pyroxenes

(olivine-bronzite

chondrites);

of ordinary

chondrites);

(3) low iron-low metal

EG-39 and EG-40 demonstrate

because

(1953) demon-

in terms of such

the existence

of two groups

(22.33 wt %), the other a

content

of olivine

chondrites:

the

(1) high iron

(H)-

(olivine-hypersthene

(olivine-hypersthene

the existence

+ \

"

in coexisting

1964) established

(2) low iron (L)-group (LL)-group

in some 800

distribution

(Keil and Fredriksson,

of three major groups

and hold only for

the L group and the H group,

they designated

of the fayalite

of metal

of the metal

chondrites)

by Urey and Craig

a low total iron content

Determination

is corre-

and since the amount the Ni content

(Mason, 1963) and the iron and magnesium

existence

Figures

rules should be considered

(28.58wt %) which

respectively.

constant,

of the metal

more of the iron is oxi-

increases,

in the Fe/Si ratio discovered

a simple model. of chondrites,

group

of the silicates

that the ordinary

high content

of oxidation

the amount of Ni remains

chondrites

strated

and the Ni content

degree

chondrites).

of these three groups

as

RAMSOORF

~ o

•

MOL"

Figure (Fe+Mg)

EG-39. in

PER

Ratios of Fe/(Fe+Mg)

rhombic

pyroxene

CENT

F, ~tlM9 IN

in olivine

for 86 chondrites

EG-58

OLIVINE

plotted

against

ratios

of Fe/

(from Keil and Fredriksson,

1964).

I

I

I

T

I

40 w

LL GROUP

I-

::J 30 )!

•• .a •

.a_

L GROUP

a

-

0

(>

w _J

-

~IO

0

Figure

EG-40.

for ordinary

obtained roxenes

0·2

Plot of mole percent chondrites

in the meteorite.

degree

of equilibration

(degree

eter classification. equilibration,

survey

stricted other

Increasing

drites

in ordinary

chromite,and

types described

variability

support

the classification exists between

hence

and pyFeo/Fe for the

(1967) introduced

and petrologic this two param-

in the degree LL chondrite,

3). feature

of whereas

in chromites

is reof the The com-

(subgroups

except

in chromite

1967).

5

for Al and Ti. in unequilib-

in unequilibrated These results

by Van Schmus and Wood

EG-59

of chemistry

in chemistry

chondrites

were observed

composition

microprobe

is not known.

of the method

(Bunch et aZ"

introduced chromite

as a function

the variability

in equilibrated

Zoning

electron

Their study, however,

(1967,1973)

the precision

variabilities

(subgroups

is also a frequent

chondrites

by Ramdohr

of chromites

compositional

chondrites

correlation

increase

out a detailed

of the chondrites.

and 6) was found to be within

rated

chemical

indicates

versus

LL chondrite.

chemistry

(equilibration)

chromite

The largest

number

ratios

does not account

and Wood

EG-4l and EG-42 summarize

Bunch et aZ. (1967) carried

to the coarse

positional

Figures

equilibrated

of chromite

and texture

Van Schmus

using two parameters:

olivines

in olivine

e.g., LL3 means low to medium equilibrated

LL6 is a highly

SpineZs.

fayalite

Feo/Fe

1967).

This classification, however,

classification

versus

of Fe and Mg among coexisting

among silicates.

of equilibration).

0'8

in olivine

and Wood,

and the mole percent

ratios

0'6

of fayalite

(from Van Schmus

from the distribution (Fig. EG-39)

a simplified

0·4 Feo/Fe

(1967).

and the classification

chonindeed A direct of

Petrologio

E

o Chemical group

H

I

I

EI_

E3

E2_

01

02

LI

C4

----

H2

----

-

----

LLI

• Number

Figure

EG-4l.

--_-

9

44

----

L5

L6

LL5

LL6

152

_18/_43

LL3

LL4

-

H6

74 ----

L4

LL2

H5

35

L3

L2

----

----

1 H4

7

6 C6

2 ----

H3

E6

2 C5

8

16

----

-------LL

E5

4

03

4

---L

£4

I'

,--------

---HI -

type

4

3

7

21

of examples of each meteorite type now known is given in its box.

Classification

of chondrites.

Number

known is given in its box (from Van Schmus and Wood,

of each meteorite

type now

1967).

Petrologic type

E

=

c

I

•

1

":

Enstatite chondritos

==1=

i

Corboneccous chondri!:.cs

_, "4

1===1

;===1'=

H

Bronzite chondrites

--'-----1-----

1

-----

Hypersthene

L LL

1----

)

t

chondrites

AmphO~OI'iC Ch0n(ll'itc~

LL3t

• Unpopulated

t Ordinary chondrites. t Unoquilibretcd ordinary Figure

EG-42.

fication

of meteorite

(from Van Schmus and Wood,

equilibrated parent

Location

chondrites.

chondrites

into H, L, and LL groups.

that FeO and Ti0

to LL groups.

contents

are well

the silicates.

of equilibrated

are compared

as mole percent

classi-

From Table EG-6 it is ap-

and Cr 0 , MgO, and MnO decrease from H to L 2 2 3 Bunch et al. (1967) did not include chromites from subgroups 3

with

classification

chemical

increase

and 4 since they were found to vary equilibrium

type in the petrological 1967).

chondrites

becomes

(Fig. EG-43).

Both FeO and Ti0

2

chromite

more evident of coexisting

In most

in chromite

EG-60

and are apparently

between

to the iron oxide content

FeO/(FeO+MgO)

separated.

in composition

Correlation

not in

composition

and

if major oxide olivine

expressed

cases, H, L, and LL groups

increase

from subgroups

3 to

Composition

Table EG-6 of chromite from equilibrated

H group

(10)""

chondrites.H

L group

(7)

LL group

(6)

56.9

56.1

A1 0 2 3

5.9

5.3

5.7

V 0 2 3

0.68

0.72

0.73

2.81

3.23

Cr20

3

Ti0

2.33

2

FeO

31.2

54.4

33.0

34.5

MgO

2.66

1.99

1.62

MnO

0.94

0.74

0.63

100.61

100.66

100.81

Total

" ""

Analyses for subgroups 5 and 6; all from Bunch et al, 1967, Table 1, p.1571 Numbers in parentheses indicate number of meteorites analysed

"

"

·. ' . "

5 1.0 1.5 2,0 2.~ 3.0 3.5 wt'~'H P(RCENT ToOl IN CHROo,UT[

'.0

b

, :~:tr

"

....

'I~a' !::,:t

·

·'..

:230

g""L ;

19.0

g

H.O

15'°0

..

ill'

...'"

..

,.~·-·;.~z~o·---t;"--)~Q---t.5

,5 WEIGHT

p(Rc(

.. r

"'qO

'''I

CHROMIT[

d

Figure

EG-43.

equilibrated

Correlation

between

H-, L-, and LL-group

of FeO/(FeO+MgO)

in coexisting

chromite chondrites

olivine,

and b) and Cr 0 and MgO decrease Z 3

composition (subgroups

and classification 5 and 6).

With

FeO and Ti0 of chromite increase 2 (c and d) (from Bunch et aZ., 1967).

EG-6l'

of

increase (a

5 in Hand

L groups.

5 to 6, whereas

However,

Ti0

tion of Van Schmus

2 and Wood

a major

chondritic

group

mineral

compositions

assemblages

(1967) implies

in H6 chondrites

indicate,

however,

a primary

origin

Spinels

classification

that relations

are consistent

i.e.,

of H3-type

(1964), Ramdohr

and mineralogical

equi-

(e.g., by slow cooling).

between

chromite

with either

composi-

a metamorphic

or

chondrites

coexist

of chondrites.

found in Ca-, Al-rich

with a Ti-rich

process

within

series,

by metamorphism

that chemical

a primary

subgroups

metamorphic

Keil and Fredriksson

during

from subgroups The classifica-

that their petrographic

were established

formation.

may well be attained

tions and subgroup

slightly

H3 through H6) represent

Bunch et al. (1967) indicate

However,

decreases

(Bunch et al., 1967).

increasing

(e.g"

after chondrite

(1967), and many others libration

FeO in chromite

continues

fassaite,

inclusions

in carbonaceous

anorthite

and perovskite

gehlenite,

(sometimes

with

Ca2 ((Al,Ti)24038) (Marvin et aZ., 1970; Keil and Fuchs, 1971). Two types of coarse-grained Ca-, Al-rich inclusions with spinel as a major constit-

hibonite,

uent were reported

from the Allende

tains 80-85 percent

melilite,

Clinopyroxene,

if present,

or surrounding

cavities

percent

clinopyroxene,

percent

melilite.

meteorite

15-20 percent

is usually

restricted

in the interior 15-30 percent

(Grossman,

Type A con-

1975).

5-20 percent

between

perovskite.

to thin rims around

(Grossman,

spinel,

The main differences

1975).

spinel and 1-2 percent

inclusions

Type B contains plagioclase,

35-60

and 5-20

the two types of inclusions

are

abundance phases

and composition of clinopyroxene. Thermodynamic calculations for 4 condensing at 10- atmospheres in the solar nebula indicate the following

sequence:

corundum,

forsterite,

Ti 0 ' 3 s

hibonite, anorthite,

perovskite, enstatite,

geh1enite, rutile,

spinel,

albite,

Fe-metal,

and nepheline

diopside, (Gross-

man, 1972). The spinel-bearing densates

are almost pure MgA1 0 2 4 et aZ., 1970; Grossman, CaO, and Ti0

in pyroxene.

Figure

are considered

occurring

Ilmenite;

in Allende

traces of FeO, Cr 0 , Ti0 , and CaO (Marvin 2 3 2 1975; El Goresy, unpublished). The Cr20 and most FeO, 3 are well below 1 percent, No systematic difference in spinels

enclosed

in melilite

and those enclosed

EG-44 displays

Snetsinger

and Keil

from equilibrated

classification:

to LL groups

to be early con-

in these inclusions

the Cr 0 content versus 2 3 in type A and type B inclusions (Grossman, 1975).

of ilmenite chondrite

inclusions

Spinels

but contain

contents 2 was found between

composition

spinels

Ca-, Al-rich

from the solar nebula.

(Table EG-7).

(1969) indicate

ordinary

chondrites

FeO increases

Ti0

2

that the average

is, like chromite,

and MgO and MnO decrease

These relationships

EG-62

content

are displayed

of

composition related

from H to L

in Figure EG-4s

to

ALLENDE SPINEL COMPOSITIONS o 5 Type A Inclusions o 2 Type B Inclusions

,.,

o

....

N

o

U

o o

1.0 WT. % Ti02 Figure

EG-44.

seldom

exceed

Cr203 and Ti02 contents of spinels 1 percent (from Grossman, 1975).

Table EG-7.

FeO

Ilmenite

from equilibrated

in Allende

ordinary

inclusions.

chondrites,~

H 5,6

L 5,6

LL 6

40,9

41.9

44.2

MgO

4.10

3.30

1.80

MnO

3.20

1.50

1. 10

Ti0

2 Cr 0 2 3 Total

"

From Snetsinger

51.7

52.7

51.7

0.09

0.31

0.31

99.99

99.71

99.11

and KeiZ, 196~ Table 3, p. 784

EG-63

They

35,

MgO

o

r

30

0

25

r

"'_ 20 0 a-

01,; ~ +

"H5. H6 o L5. L6 o LL6

00

""Q

15

... -l 0

l

~ 35

~ cr

FeO

Mno

a

30

0

0

W Q.

1

25

w

..J 0

I

&

"

,,4:>

~: I

I

0

'l>

"

43

41 42 40 4 5 3 WEIGHT PERCENT IN ILMENITE

2

Figure EG-4s. Mole percent MnO and FeO in ilmenite

I"

"

&

44

45

FeO/(Fe+MgO)

in olivine

versus weight

of equilibrated

chondrites

(from Snetsinger

percent

of MgO,

and Keil,

1969).

where

FeO/(FeO+MgO)

in olivine

ilmenite.

Although

positional

trends similar

correlation ilmenite

is plotted

the statistics

indicates

did indeed

against MgO, MnO, and FeO in coexisting

are poor due to the rarity

to those observed

in chromites

that the four phases equilibrate

olivine,

of ilmenite,

com-

can be recognized.

orthopyroxene,

The

chromite,

and

in these chondrites.

Achondrites Chromite

is the most abundant

lowed by ilmenite diogenites,

chassignites,

and El Goresy positions

and then rutile

opaque

and several

(1969), Bunch and Keil

in Angra

dos Reis

(angrite)

(1975), and Boctor et aZ. (1976),

from nakhlites

clasts in the Kapoeta Bunch and Keil

(particularly

for Ti and Al) of chromites group),

homogeneity

although

of all chromites

Lovering

he analyzed

were

Howardite

compositional

from achondrites

EG-64

com-

by Bunch

study of mineral

was recently

in the Moama

Spinel

by Keil et aZ.

also reported

(1975) indicates

folin

by Ramdohr

(1975).

reported

A detailed

(1971) report

the meteorite

meteorites, compositions

have been published

were recently

Chromian

Dymek et al, (1976),

Chromite

(1971), and Lovering

(1976),

in different

1973).

eucrites

and Reid blages

titanomagnetites

oxide in achondritic

(Ramdohr,

by

variability

(without a complete

eucrite.

assem-

published

specifying chemical

Bunch and Keil

(1971) indicate in eucrites:

some distinctions

between

in diogenites

chromites

in diogenites

and chromites

in A1 0 and MgO, whereas 2 3 chromites in eucrites are higher in Ti0 , FeO, and V 0 • Spinel in Angra dos 2 2 3 Reis is a magnesian, chromian hercynite (Keil et aZ., 1976). The howardite Kapoeta

Chromites

was found to contain

into two broad (pyroxene-

lithologic

various

Spinel

are unusually

compositions

in Table EG-8:

(1) Chromites

lation due to the strong variability and

et al. (1976)

could be grouped

(2) comprehensive of additional

enriched

of mineralogy

achondrites

in opaque

features

of the various

to those presented

are needed

can-be

do not show any corre-

and history

similar

oxides

types of achondrites

Two important

in achondrites

studies

basaltic

(pyroxene-bearing)

in various

are given in Table EG-8.

(other than nakhlites)

subclasses;

clasts which

and (b) pyroxenitic

Type b clasts

and ilmenite).

recognized

basaltic

types on the basis of modal mineralogy--(a)

and plagioclase-bearing)

(Dymek et aZ., 1976). (chromite

are higher

by Dymek

for better understanding

of phase petrology. The nakhlites presence

are characterized,

of titanomagnetite

compared

with ilmenite

to other meteorites,

and ulvDspinel

(Bunch and Reid, 1975; Boctor et aZ~, 1976).

Ilmenite

orite was found to have broken

+

report

down to rutile

lamellae

in the Lafayette

mete-

Boctor et aZ. (1976)

hematite.

that the @aximum

of about

calculated hematite content corresponds to a temperature 17 740°C and f02 of 10- . Such values are similar to those obtained for

the Skaergaard

gabbro.

from the Lafayette the original

Boctor

meteorite

magmas

et al. conclude that the phases they reported

represent

the late stages

and that the parent

body from which

may have undergone

major primary

believed

in the early crystallization

operative

Ilmenite. ilmenite dritic

by the

exsolution

Zoning

in several

ilmenite

ordinary

and minor

achondrites

is usually

differentiation

depleted

Lafayette

of a nature history

grain-to-grain was reported

of differentiation

of

was derived

similar

to that

of the lunar crust.

compositional

variability

of

(1971).

Achon-

by Bunch and Keil

in MgO and MnO compared

to ilmenite

in

chondrites.

Stony irons Chromites

in pallasites

tend to occur as coarse

(e.g., up to a few centimeters

in the Brenham

olivine

in the iron mass.

or completely

embedded

to very coarse

pallasite)

either

grains

coexisting

They are usually

with

characterized

by their low Ti0 AlZ03

content (0.18% average) but variable Cr 0 (60.5 to 69.0%) and 2 2 3 (1.5 to 9,1%) contents (Bunch and Kei1, 1971). Compositional variability

of chromites

in mesosiderites

is more pronounced

EG-6s

than in pallasites.

The Ti0

2

Table EG-8.

Composition

2

3

55.50

46.10

10.10

9.80

0,43

of spinels

from various

achondrites.

4

5

6

7

8

48.90

3.30

38.87

40.25

47.35

46.18

9,30

54.50

4.92

5.33

7.49

9.19

0.28

0,75

0.07

0.58

0.24

Cr 0 2 3 A1 0 2 3 V 0 2 3 Ti0 2 FeO

0.38

0.34

1.09

3.70

4.10

0.65

10.86

11.01

3.12

5.57

28.80

36.50

35.70

28.40

40.57

41.45

36.04

36.61

Fe 0 2 3 MgO

-

-

-

3.40

3.90

2.86

0.59

8.00

0.43

0.71

2.78

1.02

MnO

0.68

0.54

0.59

0,18

1.04

0.95

1.06

0.98

100.50

99.97

99.93

98.50

97.26

100.08

98.26

99.79

Total

1: Average chromite composition in diogenites p,

149)

(Bunch and Keil, 1971, Table 4, ,

2: Chromite composition in Chassigrry(chassignites) (Bunch and KeiZ, 1971, Table 5, p. 150) 3: Average chromite composition in eucrites (Bunch and Keil, 197" Table 4, p , 149) 4: Spinel composition in Angra dos Reis (Angrites) (Keil et al., 1976, Table 1, p , 444)

5: Chromite composition in basaltic clast A in Kapoeta Howardite (Dymek et al., , 1976, Table 1, p : 1117) 6: Chromite composition in pyroxenitic clast B in Kapoeta Howardite (Dymek et al., 1976, Table 2, v- 1118) 7: Chromite composition in fine grained pyroxenite clast C in Kapoeta Howardite (Dymek et al., 1976, Table 3, p. 1120) 8: Chromite composition in fine grained porphyritic basalt clast P in Kapoeta Howardite (Dymek et al ., 1976, Table 4, p, 1121)

EG-66

content

of chromites

in pallasites mite

compositions

mites

in mesosiderites

(Bunch and Keil, in pal1asites,

in other stony irons,

their high ZnO content

is relatively

1971).

higher

than that of chromites

Table EG-9 shows a comparison

mesosiderites

chromites

and 10dranites.

present

(0.78-1.68%).

in lodranites

In this respect,

between

Compared

are unique

chro-

to chro-

due to

they are analogous

to

chromites in iron meteorites. Compositions compositions higher

of ilmenites

in achondrites.

in mesosiderites However,

are generally

ilmenites

in MgO, MnO, and Cr203 than ilmenites

similar

in mesosiderites

in achondrites

to ilmenite

tend to be

(Bunch and Keil, 1971).

Iron meteorites Chromites with silicates troilite Goresy,

in iron meteorites in troilite

inclusions 1965).

fect euhedral

This documents

crystals

sions with silicates,

with

inclusions

the thiospinel

the chalcophile In the metal

with sharp crystal chromite

Table EG-9.

Cr 0 2 3 A1 0 2 3 V 0 2 3 Ti0 2 FeO

and graphite

it coexists

mium in the same meteorite.

occur both in the metal

exhibits

groundmass

(El Goresy,

and lithophile

groundmass

In troilite

boundaries

3

64,00

52.00

61.83

5.60

11.50

4.43

0.54

0.54

0.46

0.18

1.84

0.91

31.00

22.28

MgO

5.80

2,29

6.47

MnO

0.65

0.77

1. 10

ZnO

-

99.94

1.27 98,75

1: Average chromite composition in pallasites (Bunch and Keil, 1971, Table 7, p. 152) 2: Average chromite composition in mesosiderites (Bunch and Keil, 1971, Table 7, p. 152) 3: Average chromite composition in Lodran (lodranite ) (Bild, unpublished analyses) .

EG-67

inclu-

only against

Composition of chromites in pallasites, mesosiderites, and lodranites.

2

100.13

of chro-

always form per-

23.20

Total

In many

F'eCr2S4 (El

behavior

chromites

faces to the iron.

idiomorphic

1965).

daubreelite,

and together

troilite

but not against

equilibration

between

the early history inclusions type;

in iron meteorites

County,

Station

classifications Odessa several

members

the majority pallasitic, contain

are difficult

composition,

of iron meteorites and mesosideritic amounts

(2) Copiapo

types:

Enon,

by Bunch et al.

abundance,

ambiguous.

inclusions.

is unique

of MnO

texture,

Chromite

and

such

is common in

Chemistry

compared

on the geochemical

ZnS (El Goresy,

phile and lithophile

behavior

to chondritic, chromites,

behavior

achondritic,

Usually,

coexist with

This establishes

iron meteorites

in

they This

of both Mn and Zn in

these chromites

1965).

of these two elements

from various

of chromites

(up to 4.2%) and ZnO (up to 2,31%).

In the same inclusions

of chromites

type;

introduced

mineral

(except for Lodran)

MnS, and sphalerite,

positions

(1) Odessa

in

silicate

1965; Bunch et al., 1970) but was not found in

of the Copiapo-type

iron meteorites.

and olivine

of small sample populations,

and somewhat

(El Goresy,

again puts some constraints

groups:

The classification

Due to the problem

appreciable

pyroxene

Bunch et al. (1970) classified

into several

and Netschaevo,

and Toluca

This feature may indicate

rhombic

type; and three other "miscellaneous"

(1970) is based on mineralogical shape of inclusion.

(Fig. EG-46).

and coexisting

of the meteorite.

(3) Weekero

Kendall

silicates

chromite

alabandite,

both the chalco-

in iron meteorites.

Com-

are given in Table EG-10.

(

I

Figure

EG-46.

silicate-bearing subhedral Ramdohr,

Mundrabilla troilite

features

towards

(coarse octahedrite), nodules.

Australia.

Note sharp boundaries

silicates;' length

unpublished).

EG-68

of photograph

Chromite to troilite 200 microns

in and (from

Table EG-IO.

Composition

of chromite in various

iron meteorites.

2

3

4

Cr 0 2 3 A1 0 2 3 V 0 2 3 Si0 2 Ti0 2 FeO

69.40

71.90

71.70

68,40

2.51

1,24

0.42

10.90

0.31

0.26

0.57

0,68

1.01

0.40

0.48

0.02

7.00

12,60

15.10

2.50

MgO

16.00

10.20

7.10

14.20

MnO

2.22

2.28

3.40

4.20

ZnO

1.37

1.39

1.70

0.02

100,03

100.27

100.47

100.88

0.21

Total

1: Average of 25 grains in Mundrabilla (Ramdohr et al., unpublished data). 2: Odessa chromite (Bunch et al" 1970, Table 7, p, 314) 3: Copiµp.o chromite (Bunch et al., 1070, Table 7, p. 314) 4: Kendall Count¥ chromite (Bunch et al" 1970, Table 7, p, 314)

The very high Station,

Cr 0 content (except in Kendall County) is striking. 2 3 Colomera, Kodiakanal, Enon, Kendall County, and Netschaevo,

are characterized 17.6%).

Figure

In Weekero chromites

by their relatively EG-47

high A1 0 content (between 2.68 and 2 3 2 the Fe+ /(Fe+2+Mg+2) in olivine versus Fe+2/

displays

0,5 c: '" :~ 0,4

.Chassigny

0

='" c:

0,3

",LL "'L "'H

::E

0;.

0.2

D

'"

u,

~

O,lr

Mundrabilla

•

_._ 0,2

PallasitesD Silicate 'Y'dI!I 0Netschaevo Inclusions

e

II

rib 0,4

0,6

0,8

1.0

Fe·'/(Fe·'.Mg) in Chromite Figure

EG-47.

Fe+2/(Fe+2+Mg)

in olivine

Chassigny,

LL-, L-, H-chondrites,

meteorites

(modified

versus

pallasites,

from Bunch et al., 1970).

EG-69

2 2 Fe+ /(Fe+ +Mg)

silicate

in chromite

inclusions

in iron

in

(Fe+2+Mg+2) silicate

in coexisting

inclusions

meteorites

chromite with chondrites,

in iron meteorites.

show the lowest

Also evident

LL, L, H chondrites, Rutile

or with

is usually

Ti02 Nb0

2 FeO

MgO MnO Cr 0 2 3 A1 0 2 3 V 0 2 3 Total

nodules

to rutiles

1965). (Busek

(1971) found that rutile

in iron meteorites

(Table EG-ll).

Composition of rutile in iron meteorites and Vaca ~Iuerta mesosiderite, ~

4

95.06

95.10

92.48

95.58

2.93

2.89

1.63

0.38

0.73

0.10

1.00

2.27

Fe3+, or to the rectangle on the right if Fe3+ > AI; this second plot is specific to the divalent ion The scale values represent the number of cations in tetrahedral and octahedral coordination for the spinel formula based on 32 oxygens and 24 cations.

Hg-105

Figure Hg-2s Mineral

(a)

Dark gray chromian-spinel along the peripheral

(b)

Euhedral

primary

core mantled

margins

clusion with an attached

by titanomagnetite

to titanomaghemite

pseudobrookite

ilmenite

Morphology

crystal

crystal

containing

Skeletal

(d)

An ilmenite crystal with glass inclusions

(e) - (h)

with glass and finely

crystallites

Skeletal

growth morphologies

trend towards euhedral

a cylindrical

crystalline

silicate

inclusions.

glass i

second

0.09 mm. generation

growl

0.09 mm.

of titanomagnetite

morphology.

oxid

0.09 mm.

and with renewed

along the basal plane.

is partially

0.09 mm.

of chromian-titanomagnetite.

(c)

of T-shaped

which

(white).

illustrative

This trend is classified

of a progressi'

as the crueifo~

tYPl

150 µm. (i)

This titanomagnetite growth, parallel primary

(j)

cross-arms

Titanomagnetite

(k) - (m)

terminations, 1

NOTE:

=

(cf. with e-h).

crystal

Titanomagnetite

gonal multiple

0.12 mm; m

is also of the crucifo~

crystal

to {Ill} spinel planes,

crystals

=

cross-arm

type.

80 µm.

of the complex type contain

Growth patterns

and along primary,

tl

length of tl

0.11 mm.

of the multiple

cross-arms.

type with the distinction

takes place along the entire

secondary

orthogonal

and non-ortn

are evenly or haphazardly or tertiary

cross-arms.

initiated k

=

at

0.13 mm;

750 µm.

The scale given after each caption or sets of captions is equal to the width of the photomicrograph in micrometers, or mm. All plates are in reflected light oil immersion (Figs. Hg-2s-36).

Hg-I06

Figure

Hg-2s

Hg-l07

Oxidation Textures

or reduction

produced

"exsolution"

by these mechanisms

growths

formed by exsolution

cooling

a one-phase

Oxidation

"exsolution"

spinel planes

blages

Reduction

for members

without

inter-

do not form, however,

by

is stable.

of Ilm-Hem

along {Ill} cubic ss is limited to Ilm-Hemss planes. assemblages

intergrowths

designation

which

result

from

at high TOC and f02· sense to denote the

are used in the broadest

or identification

series and for members

ss

reaction.

oriented

"exsolution"

ilmenite

the specific

of the Ilm-Hem

of subsolidus

a solvus

is used to describe

and metatitanomagnetite

of oxidation

textures

ss along {0001} rhombohedral

of titanomagnetite-ferrian

Metailmenite

These

to the formation

•

ss oxidation

The term pseudomorphic

from processes

into a P-T range where

of Usp-Mt

of Usp-Mt

the decomposition

above.

is restricted

by oxidation

with the formation

detection

as defined

solid solution

results

are akin to the crystallographically

of phase assem-

of the Usp-Mt

series,

ss

respec-

tively. Maghemitization results

is restricted

in titanomaghemite

Oxide assemblages

The crystal

habits

of chromites,

in Figure Hg-2sa-m

in Figures

Hg-26-36.

exhibit

Hg-2sc-d)

These minerals

euhedral displaying

through

most prominently

developed

is normal

basal planes

(Fig. Hg-2sb)

skeletal

these may be classified of a simple axes; growth continue

entire planes;

length

cross-arms

are neither

the cross-arms

orthogonal

in Figures

lites which

are attached

the entire

length

on titanomagnetite multiple

cross-arm,

the direction

of most rapi along

{0001

are varied but type consists

to the crystallographic develop

An alternative

crystallization

along directions

of

The

and in titanomagnetites

of these arms and arrow-heads

cross-arms

forms typical

is more typically

correspond

(Fig.

respectively.

(i) the cruciform

forms:

All

which

variation

extending parallel

to

along the to {Ill} spinel

cross-arm type is shown in Figure Hg-2sj and in this type the nor is there a preferred

or at the extremities

illustrated

symmetry,

for titanomagnetites

(Fig. Hg-2se-h).

in Figure Hg-2si with

of each of the primary

(2) the multiple

COmmon

ilmenites

and lath-shaped

nucleation

rocks and the

sequence.

and titanomagnetite

forms, and with

For ilmenites

at right angles which

at the extremes

is illustrated

(Fig. Hg-2sa)

are

this section

oxides in igneous

in plate-like

growth patterns

into the following

is initiated

throughout

crystallization

and orthorhombic

secondary

to grow until all sets coalesce

this pattern

low TOC and

and titanomagnetites

forms are seen in ilmenites

Crystal

set of cross-arms

octahedral

rapid chilling.

to the c-axis, whereas

(Fig. Hg-2sd).

primary

with chromites

with rhombohedral

from lavas which have undergone growth

are the major

cubic or modified

crystals

ilmenites

are illustrated

is the typical paragenetic

characteristics

and psuedobrookites

sections

pseudobrookites,

and other examples

they are listed

(Fig. Hg-2se-m)

at relatively

ss

mineral morphology

summarized

minerals

of Usp-Mt

and textures

Two-dimensional

order in which

to the oxidation

(Fig. Hg-23b).

Hg-2sk-m

of the cross-arms;

and these are characterized

to a central

stem~omwhich

of the stem at fairly regular xenocrysts

intervals.

are rarely observed

Hg-I08

of growth

by dendritic

cross-arm

as shown in Figure Hg-2sm.

and complex,

pattern

and (3) the complex

growth

along either types are

arrays

of crystal

is initialized

This type is commonly The three types,

along

observed

cruciform,

in the same lava flow and although

the onset of co-crystallizing

silicates

the systematics

remain

to be established.

Chromian spinelss Representative are listed

are illustrated

Chromian

spinelss' which

Hg-26 and Hg-27;

but which show extensive

above the prism base.

These

compositions

spinels

commonly

which

settings observed

form exsolution

bodies respec-

are typical

is particularly

(Figs. Hg-26a-f)

sense to denote

solid solubility

and for limited

may be identified

This feature

For basalts

other examples

commonly

as defined here, is a term used in the broadest

are Cr-rich

in many cases chromian zoning.

of geological

and assemblages

below.

(Fig. Hg-24),

complex

from a variety

are shown in Figures Hg-29 and Hg-33,

the base of the spinel prism

berlites.

spinels

of the textures

and in titanian-chromites

and are discussed

compositions

of chromian

and examples

in Figures

in pircoilmenite tively,

compositions

in Table Hg-20,

among members

solid solubility

of mafic and ultramafic

optically

prevalent

because

suites and

of extraordinarily

in basaltic

the cores of crystals

on

with members

suites and in kim-

in the groundmass

are most

enriched

in Cr203, A1203, and MgO whereas the mantles are enriched in FeO, Fe 0 , 2 3 and Ti02; these distributions are apparent in the electron microprobe element distribution x-ray scanning images shown in Figures Hg-26c-f for a chromian spinel core mantled by titanomagnetite Hg-26a.

of the type comparable

These mantles

liquid and contrast where

with those chromites

the silicate-enclosed

mantles

to the assemblage

result by the reaction

crystals

at all, as illustrated

which

commonly

are included exhibit

in Figure Hg-26b.

ilmenite

phase can also become

as shown in Figure Hg-26i.

Hg-2sa

in either olivine

the cores for nucleation

In rare instances

or pyroxene

mantles

the basaltic

(Fig. Hg-26g-h);

and

with an Fe-Ti rich

very thin reaction

For kimberlites,

early Cr, Mg, Al and later Fe + Ti is also observed cally later mantling

shown in Figures

of early formed chromite

or no trend of

but the paragenetiand growth of picro-

for groundmass

spinels, but commonly rims, the cores show a preferred enrichment of Mg, AI, and Fe3+, with 2 zones which are enriched in Cr and Fe +, and with outermost zones which are

in garnet kelyphitic intermediate Ti enriched.

These complex

mass crystal,

and in Figures

Figure Hg-27 chromites

olivine

illustrates

the effects

by titanomagnetite in Figure

titanomagnetite tinization

Hg-26k

Hg-27;

in partially

of magmatic

chromites

glass and sulfide

coprecipitating

at high temperatures

(Figs. Hg-27e-f);

of these overgrowths

oxidation

literature

is in contrast

4.

assemblages

1968; Beeson,

Neither

associated

trends

of chromite

1971; Springer,

1974; Hamlyn,

1975; Bliss and Maclean,

1975).

nor the

attention

to titanomagnetite

1974; Engin and Aucott,

(e.g., Onyeagocha,

discus-

are to be

characteristics,

1976) and of the alteration

and magnetite

+

with serpen-

Additional

spinels have received

of chromite

Hg-27b-c;

growth of chromite +

and of the oxidation

of chromian

to the reaction

are in Figures

in Figures Hg-27g-h.

the resorption

and

are shown in

(>600°C) of chromite

and the alteration

harzburgite

and silicates,

inclusions

and resorption

high-temperature

Evans and Moore,

between

for a ground-

in kelyphite.

corrosion

found in Table Hg-19 and in Chapter

which

in Figure Hg-26j

in Figures Hg-27a-c;

decomposed

sions of the relationships

of textures

Silicates,

the oxidation

in basalts

are illustrated

and I for two crystals

a variety

and titanomagnetites.

Figure Hg-27a; mantling

distributions

in the

(e.g.,

to ferritchromite 1974; Ulmer,

Figure Hg-26 Chromian (a)

Asymmetrically mantles

Euhedral

discrete

crystal

spinel core mantled

crack and in patches

cluster partially

and the intermediate

as the cruciform

classified

ss

chromian-spinel

are titanomagnetite

The adjacent

(b)

mantled,

Spinel

of titanomagnetite,

in olivine.

Th,

although

incomplete,

would be

0.16 mm.

type.

by titanomagnetite.

associated

enclosed

zones are chromian-titanomagnetit,

with inferred

Mtss are also present

cracks below

the polished

along the surface.

0.16 mm. (c-f)

X-ray scanning

images obtained

of major elements

(g-h)

by electron

microprobe

spinel-titanomagnetite

illustrating core-mantle

displayed

are Fe (in c), Ti (in d), Cr (in e) and Al (in f); the core is thus virtl (d) and the mantle

core contain

Fe (c).

Multiple

(i)

Euhedral

(j)

Oscillatory darker

virtually

magnetite

Cr (e) and Al-free;

The elemel

both the mantle

and

from kimberlites

with chromite

cores and mag·

0.15 mm; h = 0.08 mm. core, epitaxially

overgrown

by picroilmenite.

0.09 mm.

3 in spinel where the white areas are Mg, Al and Fe + enriched, 2 areas are Cr+Fe + enriched, and the mantle is Ti enri~hed. zoning

Compositionally rims associated

I

O.ll mm.

zoning in chromian-spinels g

to concentration.

TI

of the spots are approximately

ally Ti-free

proportional

the distribut:

relationship.

intensity

netite mantles.

(k-l)

for a chrome

similar

to (j), but these spinels

with Ti-phlogopite.

are present

k = 0.09 mm; 1 = 0.07 mm.

Hg-110

tl

in garnet kelyphit,

Figure Hg-26

Hg-lll

Chromian

(a)

Glass inclusions island

(b)

0.14 mm.

Coarse web-shaped

chromian

edges. (c)

internal

later chromite

(d)

glas

is a function

morphology

an

mantle

and dominantly

mantle

cores have a similar

core of chromian

and an outer margin

in the core are also mantled ss

The lighter

crystalline,

of the titanomagnetite

and that the internal

by titanomagnetite;

cO,rrosion.

of

to the out,

0.16 mm.

A symplectic

Mt

spinel mantled

magmatic

spinel with partially

Note that the extent

size of chromite

chromian

in the glass suggests

titanomagnetite.

Reactions

ss

in a phenocrystic

of chromite

inclusions.

Spinel

spinel mantled

by successive

of titanomagnetite;

and the smallest

barriers

most of the cuniform

areas contain

the largest

of segmel

mantles

0

O.ls mm.

Euhedral

chromian

overall

spinel

outer-morhpology

core mantled

by subgraphic

of the symplectite

chromite

is broadly

+ glass + olivine.

similar

to that of the cor'

0.16 mm. (e)

Irregular

glassy inclusions

gone oxidation

"exsolution"

in chromite

mantled

and subsequent

by titanomagnetite

partial

decomposition

which has unde

+

to R

Hemss

0.16 mm. (f)

The dark central magnetite, oriented residual rods.

(g-h)

spinel,

the outer assemblage

is oxidized

titano

Ilmss but are now R + Hemss The Ii: {Ill} trellis lamellae were also originally Ilmss but are now R + Hemss; host Mtss are still apparent, and these contain dark, oriented pleonaste attached

areas were

0.15 mm.

These during

core is chromian

and the white

two examples

illustrate

the serpentinization

Fe3+-rich

chromian

(see Table Hg-2l).

the features

of chromite

magnetites,

typical

in ultramafic

and the exchange

O.ls mm.

Hg-1l2

of "ferritchromit" rocks.

chemistry

which

res'

The outer mantles

is complex

and varied

a'

Figure Hg-27

Hg-1l3

Figure Hg-28 Ilmemite-Hematite

(a-b)

The light-gray

hosts are titanohematite

ferrian-ilmenite. centrations

In (a) individual

of ilmenite

and these preferred lenses

by a depletion

from very extensive

second

generation

migration

of exsolved

is the synneusis

texture.

(e-f)

Low and high magnifications, lIm

and with mantles

ss of pleonaste

which

The outer margins

result in a symplectic

suggesting

the result of exsolution e

=

0.15 mm; f

The reverse solution.

=

a

contin-

cores with exsolved lenses ss to these lenses are coarse lamellae

intergrowth

that each of the pleonaste

that the spinel predated

contain

are in optical

to

has re

of Hem

In addition

ss share the same plane of exsolution

cation it is evident

in Ilmss lenses

0.15 mm.

respectively,

of lIm

0.15 mm.

of ilmenite

These mantles

and for all sets which

is {0001}.

the large lenses

to have formed at TOC lower than

during exsolution.

Hemss'

are rounded,

Each of the larger ilmenit

The cores of these Hem grains contain similar distributions ss those shown in (a-b), but here the development of thick mantles suIted

lenses are and large con

In (b) the grains

zone, and the regions between are assumed

This distribution

uity, the plane of exsolution

(g-h)

darker-gray

sharp terminations

are not as evident.

by finer lenses which

those of the larger bodies. (c-d)

and the oriented

grains show

at the grain boundaries.

concentrations

is surrounded

are occupied

ss

as those of the ilmenite. with Mtss.

lamella

Ilmss exsolution.

or an exsolution-like

0

process

At high magnifi-

is surrounded Whether

by Ilm

these spinels

is not clearly

' ss are

understood.

400 µm.

relationships

are illustrated

The plane of exsolution

here for ilmenite

is that the lenses are extremely

fine grained

non-exsolution,

along the grain boundaries.

are concentrated

hosts and Hem

is still {0001} but the notable

Hg-1l4

and that depletion

exss difference here

zones, or zones

0.15 mm.

0

Figure

Hg-28

Hg-lls

Figure Hg-29 "Exsolution"

(a-b)

Discontinuous Both grains boundaries

oriented

distribution

but the rods are uniform

Although

to {0001}.

the oriented

optical

contrasts

lamellae

in both

(c)

lamellae

with respect

(e-h)

lighter

lamella

is present

in these picroilmenites

process

related

are enriched

oxidation

and the surrounding in grain

A picroilmenite

differen

to their hosts,

the lighter

in Mg and AI.

lamellae

The bleached groundmass

the

are high zones i

crystals

(d) and a small amount

the darker

The ilmenite

exsolved

gray lenses

in the typical blitz

are rutile

in (e), along which abundant

a

is also

are Ilmss;

texture.

Ilmss have exsolved;

t

One ruti

no 11m is

plane is {0001} and the rutile exsolved

to be {Olll} and {0112};

the former results

by exsolution

sens

the latter is more likely the result of an "exsolution"-like to oxidation.

A rutile phenocryst and perovskite.

is assumed

have distinctly

to each other and with respect

rutile is present

gray lamellae

are assumed

stricto, whereas

picroilmenites

to Ilmss grain

(c). O.ls mm.

in grain

in (g).

in kimberlitic with respect

The plane of exsolution

The host in these grains is titanohematite;

planes

(j)

of partial

Peripheral

z-shaped

present

(i)

in size.

grains are Mg-Al-titanomagnetites;

are the result

present

chromites

of lamellae

and Rutile

0.15 mm.

in Fe3+, and the darker lamellae

perovskite.

Hematitess'

rods of titanian

show a varied

to be parallel (c-d)

in Ilmenitess'

containing

0.08 mm. sigmoidal

lenses

of Ilmss and mantled

by Ilmss' Mt

0.15 rom. crystal with a core of rutile;

sions of Ilmss rather gina 1 and groundmass

than the oriented

arrays

grains are perovskite.

Hg-1l6

the rutile typical

0.15 mm.

contains

irregular

of the association.

incl The m

Figure Hg-29

Hg-1l7

.1. ""f/f:::.ff,"{.;,

I.A::;.-rt.t;;IIIUvv

"t;:;

SS

A review of the textural are illustrated Hg-28,and

in Figures

assemblages

Hg-28-3l.

the experimental

associated

Exsolution

with members

of the Ilm-Hem

characteristics

phase relationships

are discussed

are illustrated

in Chapter

series ss in Figure

2 for the series

Fe 0 -FeTi0 . Exsolution of Hem from Ilmss' and of Ilmss from Hemss is restricted to 2 3 3 ss deep-seated intrusions and are particularly characteristic of anorthosite associations and other basic suites, but are also present is parallel

to the {0001} rhombohedral

decomposition

as a consequence

distribution

(host-dissolving

(exsolving

medium).

of the solute migrates common,

although

towards

a process

between

of extreme

crystal boundaries

the resolution

exsolution

The plane of exsolution takes place in accord with

and solvi-intersection.

phase) distributed

The growth and

texture,

thicker

and atoll textures

the primary

which yields

successive

i.e., with finer

lenses

and prolonged

of these photomicrographs,

bodies within

of exsolution

suites.

results in a synneusis

Under conditions

not within

some cases tertiary suggest

of slow cooling

of large and finer lamellae

lenses of the solute

in granitic

direction, and exsolution

lamellae;

in the solvent

cooling,

diffusion

result.

Equally

are second and in

these relationships

sets of compositional

pairs of

Ilmss and of Hem

which conform respectively to conjugate sets of lamellae whose composs are controlled by the slopes and the limiting boundaries defined by the immiscible

sitions solvus

region.

and by Ramdohr by Carmichael natural

Textural

(196l,1962).

material,

in Chapter

2.

Hem

'

A detailed

and McNutt

with initial

the host and Ilmss members

by Carmichael

they suggest

that exsolution

which are more enriched

of compositiol

is consistent

as there is a greater preponderance

solute;

solid solution

for the reverse

relationship

Other examples

titanohematite. acid suites. lographic

of previously

the exsolving

These examples The exsolved

control,

assumed exsolution

solutes are spinels are typical

forI

constituent

are shown in Figure Hg-29, and in

from picroilmenite

and of rutile from

of Ilmss in kimberlites,

phases have an exsolution

and with respect

of dis-

members

of Ilmss hoStl

Hem solute is a relatively evenly-distributed single generation ss for example, Figs. Hg-28g and Hg-28h with Figs. Hg-28b and Hg-28f).

these crystals

in

with the

the exsolved (compare

(1961) on

(1973) is discussed

compositions

lenses in grains where hematite

the exsolved

(1965)

in the series has been pub-

This interpretation

in Figure Hg-28 inasmuch

or second generation

mechanisms

by Edwards

of the series are discuss,

were established

of exsolution

discontinuously.

properties

solvus by Lindsley

(197l) in which

whereas

proceeds

illustrated

continuous

Solvus relationships

treatment

is continuous,

8s exsolution

ss textures

of the series are considered self-reversal

and the revised experimental

lished by Kretchsmar with >Ilm

interpretations

(1969), and the magnetic

appearance

to the morphology

and of Hem

with respect

of crystal

lamellae.

in more ss to crystal-

However,

for

both the Ilmss and the Hem

the solvent and the solute have differing crystal symmetries ss the host nor the exsolved phase form members of either a continuous or a

and neither discontinuous

picroilmenites Hg-29b-d);

solid solution

these phases

with the parallel

stricto).

The exsolved

planes

many finer lamellae

ss

which

the paragenetic

exhibited

(Fig. Hg-2ge-h),

of the kimberlitic

and Mg-Al-titanomagnetites

along {0001} rhombohedral

of exsolution

to rutile.

constituents

(Fig. Hg-29a-b),

are oriented

In the case of Hem

lenses in addition

Hg-29f

series.

are titanian-chromites

planes which

in Ilm-Hem (i.e., exsolution sensu ss two of the crystals shown contain Ilmss

The grain in Figure Hg-2ge has a single

are clearly earlier

relationships

than the exsolution

are not definitive Hg-1l8

(Fig.

is consistent

thick lamella and

of Ilmss

In Figure

but here rutile is in far

with well-defined crystallographic control along {Olll} rhombohedral planes ress suIting in Ramdohr's (1969) "blitz" texture. The abundance of rutile in these two latter

in Hem

instances

is far in excess

the join Fe20 -FeTi0 , 3 3 most likely the result

of the limits

Hg-29i

relationship

of whether

Fe2TiOs-FeTi20s-MgTi20s'

solution

from kimberlites.

or an exsolution-like

For both grains

process

this bears a much closer relationship

trations which are likely

malcolites

along

exsolution from Usp-Mt members. ss in rutile hosts is illustrated in Figures

to develop is invoked

if decomposition as the precursor

to the expected

of a Pb

member,

ss to such bimodal

(i.e., compositions

have been recognized

the perplexin

is responsible

by the fact that rutile and Ilmss are at times present

equal concentrations;

such compositions

grains

true exsolution

fabric is compounded

by oxidation

of Ilmss lamellae

and j and in two groundmass

problem

of Ti02 in members

of Pbss members. Therefore, these assemblages are of an exsolution-like process which is related to oxidation in mucn

the same sense that Ilmss "exsolve" The reverse

for the solid solubility

in the absence

for this

in approximate modal concen-

in the system

assemblages,

approaching

and sinc

those of lunar ar-

(FeMg)Ti20s (Haggerty, 1975), the likelihood of decomposition rather than exis favored. For the cases of minor Ilmss in rutile the probability of unmixing

from a system with limited temperature

solid-solubility

and homogeneous

one-phase

may still be possible

mineral.

The mechanisms

from an initially

high

remain unresolved.

Reactions involving ilmenite-hematitess In contrast reactions oxidation textures

to the uncertainties

and the resulting and reduction derived

aired above,

assemblages

are reasonably

which

the effects

well defined.

from each of these modifying

of magmatic

and metasomatic

form from early and late stage deuteric Examples

processes

of the assemblages

are illustrated

and the

in Figures Hg-30

and Hg-31. The first process the following enriched skite

constituent:

(CaTi03);

reactions

or tectonically development

of sphene

reaction

of residual

examples

deep-seated

in Figure Hg-30.

metamorphic

intrusives

terrains,

Sphenitization

but in many underformed

and in recent hypabyssal

suites,

the

as a reaction

product of rlm or of titanomagnetite (discussed ss that it constitutes a product of autometasomatism by

liquids with early formed Fe-Ti oxides.

are in melilite

spinels

show variable

is restricted basalts,

to undersaturated

kimberlites,

The formation

of primary

suites, and the most spectacular

and in carbonati~es.

For the latter two

in MgTi03 and decomposition results also in the formation of some of which are magnesian-titanomagnetites in composition while

enrichment

reactions,

For perovskite

in Cr 0 , A1 0 and FeO. In the cases of sphene and 2 3 2 3 metasomatic ions are Ca + Si, and Ca, respectively.

the additive

reactions

Fe from the ilmenite

is accounted

and in low abundance

excess Fe is restricted absent

Ti-

(CaTiSiO ); (2) the formation of perovs of aenigmatite (Na2FeSTiSi6020). Each of these

Ilmss are enriched

perovskite

spinel,

which falls into at least

by the major newly developing

of sphene

low-grade

suggesting

perovskite

Mg- and Ti-rich others

metasomatism

characterized

in sets of photomicrographs

in regional

undisturbed

is pervasive,

and secondary

suites,

and (3) the formation

common

is magmatic

categories

(1) the formation

is illustrated

is relatively

later)

to be considered

three distinct

in analyses

and phyllosilicates

levels of Fe in CaTi0 . 3 to the very limited formation

of CaTiSiO ' s are usually

and the formation regarded

of Hem

of a

for sphene reactions ;

ss of associated

as the potential Hg-1l9

for in the formation

However,

Fe is most commonly late-stage

mineral

amphiboles

sinks that develop

the

Ilmenite

(a-b)

Sphene

The variation in crystal (c-f)

phase

orientation

in the marginal

dark gray, and perovskite Hem

ss

(g-h)

+ R are associated

The grains illustrate which

appears

crystal,

affected.

of picroilmenite

the replacement

grain

by aenigmatite

in (a) results

illustrated

0.15 rom; (e)

=

Grain

O.ls mm.

Hg-120

the spinels decomposition

are (f)

(Na2FesTiSi6020)

(g) is a discrete

Ilmss lamellae

as cossyrite)

from variations

750 µm.

ilmenite

in Mtss and an ex-

Note that it is only the Ilmss which

(also known

is

in (b).

in this series results

of Ilmss by aenigmatite

(h) has {Ill} oriented

rutile

0.15 mm.

With more intense

=

grains;

from ilmenite

Mtss + perovskite;

of Mg-rich

as the dark gray constituent.

whereas

evident

pleochroism.

(CaTi0 ) is white. 3 phases. (c, d & f)

in both

are exsolved

ss is particularly

formation

ternal composite ilmenite. replaced

lamellae

and reflection

decomposition

Reactions

of Ilmss are illustrated

in (a), and Hem

in color which

The progressive initially

replacement

(CaTiSiOs)

an associated

ss

is selectively

and that the Mtss

remains

un-

Figure

Hg-30

Hg-12l

In many cases, however, adjacent bearing

the Fe-exchange

to individual minerals,

from the system

crystals,

process

which may be possible in aqueous

is not evident

in the metasomatized

so that the Fe is either redistributed

solutions

at elevated

temperatures,

if the reactions

halos

among other Fe-

or alternatively

removed

take place at correspondingly

lower

temperatures. Aenigmatite problem

reactions

differ from those of sphene and perovskite

of Fe reconstitution

of the ilmenite

constituents

in thick differentiated of aenigmatite replacement grain

(Lindsley,

Reduction

of Mtss from Ilm-Hemss in paragneisses lamellae

of the Franklin,

experimentally

spread occurrences

evidence

post-dates

reduction.

that aenigmatite

gneisses

develop

ss of the ilmenite;

to the basal planes

and Lindsley,

rocks are in trachybasalts (Haggerty and Wilson,

of Mtss are partially

for the trachybasalts

cannot be give~ but the widespread suggests

Apart berlitic example examples

mechanism

picroilmenites

as discussed

are for ilmenite

these kimberlitic

occurrence

reduction,

above with reference

and contrast

is highly

Mg-rich

during

deuteric

reduction

ilmenites

are shown in Figure Hg-3Ic-d.

in replacing

virtually

to

and those in andesites

and the reducing

to note:

would tend to favor a subsolidu,

common in the Peru andesite apparent

suite but extremely

reduction

modifications

still exists

but to a process

(1) that oxidation

that coexist with Ilmss having reduction

lamellae

'

ss and

to coexisting

•

spinel

ss

in the former that the

of low-temperatul

exsolution of Mt

rare in kimberlites;

Hg-122

Thes,

discussed

' with a tendency ss The overall similarity

agent is perhaps more clearly defined the possibility

It is relevant

ilmenites

the Mt

untouched.

reduction

are no visibly

of these intergrowths

with the groundmass

selective

ilmenites

to CO:C02 equilibria. However, of Mtss is not related to subsolidus

weathering.

the reduc-

An explanatiol

are also present in kimss to Figure Hg-29a-d, and a second

formation chemical

of the

spinel

and is related

Ilmss in Mtss'

grains shown,

to titanomaghemite.

may have resulted

pipes of west Africa

xenocrysts

Surface weathering

mechanism,

Examples

could also be invoked which would relate

of subsolidus

from the deeply weathered

to leave the more resistant

reduction

(Haggerty et al. ,

from Teneriffe

activity.

from rhese examples

previously.

between

that this assemblage

the texture has

unpublished).

oxidized

in the Peru andesites

and

as lenses or

1964), and the only known wide-

in Figure 3la-b, and in the three ilmenite

sulfide hydrothermal

is a late-stage

from the Adirondacks

Magnetite

latter are illustrated

an alternative

data

below

These authors have noted the occurrences

from Peru

cooling;

stability

are illustrated in Figure Hg-31. ss (1964) have discussed the formation

granite

New Jersey area.

(Buddington

in igneous

lamellae

the oxidation

with experimental

have a maximum

1966) and in andesites

tion exsolution

of aenigmatite

of rlm

and Lindsley

of rlmss in hornblende

along planes parallel

been simulated

zones

crystallizatio

Examples

This is consistent

and the oxidation

by subsolidus

reduction

and

product.

et al. (1963) and Buddington

of subsolidus

in pegmatoid

replacement

compositions

1971), and with the textural

"exsolution"

of ilmenite

and in the case of the titanomagnetite

it is clear that ~enigmatite

liquid reaction

replacement are observed

host rock compositions.

in Figure Hg-30g-h

of Ilmss from the titanomagnetite.

interstitial

Buddington

Aenigmatite

and these zones in common with the primary

which show that synthetic

as the

for, and in the sense that the metasomitizing

titanomagnetites

are typical of peralkaline

for aenigmatite 900°C

of oxidized

basalts,

are illustrated

(Fig. Hg-30h),

exsolution

is accounted

than Ca + Si or Ca.

ions are Na and Si rather

insofar

lamellae

of

are moderately (2) that there

.. -------

-----------

For Ilmss in other igneous tablished,

--

----S8

-/:"---

rocks the progressive

.......-

........

IJ

.............

sequences

..._

...

..LO

~

U..LO

.....

of oxidation

and in rock types other than those of the extrusive

u;;:,,:,cu

..1.1.1.

\..IlldYLt::1.

are less well es-

basic suite,

the prevalent

is Hemss + Ti02• Examples in which the Ti0 polymorph is rutile 2 (based on x-ray data) are illustrated in Figure Hg-3le-g for three grains from a gabbroic

decomposition

assemblage

anorthosite;

additional

data) are illustrated kimberlitic

examples

in which the Ti0

polymorph is anatase (based on x-ray 2 for a progressive sequence of oxidation in a

in Figure Hg-3lh-j

ilmenite

megacryst.

Adjacent

to the ilmenite

the assemblage

is finely tex-

and is anatase + Hemss (h); in an intermediate zone the Hem is progressively ss removed and coarse crystals of anatase result (i); at the outermost grain boundary the tured,

Hemss

is largely

This example temperature

removed

and a coarse cavernous

is most typical magmatic

of kimberlitic

fluidal

interaction

network

of anatase

is the end product.

Ilmss and is most likely

and later supergene

the result of low-

dissolution.

Ulvospinel-magnetitess The consolute

point for the solvus of the series Fe Ti0 -Fe 0 is at approximately 2 4 3 4 along this join are sensitive to TOC and f02 conditions in magmas

600°C and compositions which

co-crystallize

Chapter

2.

hence,

though

of the Fe203-FeTi0 solid solution series as discussed in 3 low temperature of the critical point yields compositions which

The relatively

may be quenched suites;

members

from above 600°C in most extrusives compositions

the extremely

which

low f02 values

dictates

that it is relatively

cooling,

however,

solvus

is parallel

trated

Examples

examples,

by Ramdohr

(preprint),

there is a clearly

along the join and deep-seated

suites. by Jensen

Hypabyssal

occurrences

compositions

and exsolution

The examples

illustrated

an associated

"exsolution"

example,

to quench in members

In samples

(1966), Morse

exsolutl.on of and anorthositic (1962) and

with intermediate

constituent

intrusion,

from both localities

and the relative

parageneses

Labrador, Ilmss are

between

of USPss'

or of Mtss' and of USPss oxidation to Ilmss are variable. For in some samples Ilmss predate Uspss exsolution, and oxidation must therefore

have taken place above 600°C; in other instances of the Ilmss' which

These

and Heikkinen

in Figure Hg-32 are from the Kiglapait

stock, Rhode Island.

oxidation

of the gabbroic

de-

(1954), Nickel

(196s).

between

or

are illus-

of those examples

e.g., Anderson

by Vaasjoki

nucleation

is inhibited.

and from the Cumberland

exsolution

form when the

cloth, parquet

and Phillips

relationship

basic intrusives

even dykes appear

unmixing

relationship

(1962), and Tsvetkov

have been described

(1966) but in general

as woven,

are most typical

defined

region,

members

Uspss and of the reverse

Vincent

al-

Fe Ti0 2 4 of slow

and the plane of exsolution

are described

and Heikkinen

possible

Under conditions

solid solution

(1953,1969),

hypabyssal

of stoichiometric

immiscible

and from the reports by many other investigators,

and Stoiber members

Textures

These photomicrographs

(1960), Vaasjoki

mineral.

in exsolution

of Mtss exsolving

in the literature

(1958), Vincent

for the formation

and Mtss-rich

This results

in Figure Hg-32.

and in some high-level series are theoretically

the low temperature

to {100} spinel planes.

lit-par-lit.

scribed

within

into Uspss-rich

is intersected.

required

rare as a discrete

for members

or phase exsolution

span the entire

and oxidation

are both more highly

was hence below enriched

the formation

600°C.

in Usp

Exsolution

of Uspss predates produces

that

two constituents

and therefore FeO, and a second phase which is more highly enriched in Fe304 and there;~re in Fe3+ The former is more susceptible to oxidation and the latter consequently less susceptible to oxidation than the original Hg-123

Figure Hg-3l Ilmenite

(a-b)

The hosts

in these grains

are Mtss which

(c-d)

the cores abound with lamellae

whereas

result by subsolidus

and subsequent

reduction

the margins

are lamellar-free.

low TOC oxidation.

are Mg-titanomagnetites.

The dark gray host is ilmenite, gray sigmoidal

so that although

phase

the texture

A core of Hem

ss

with oriented

the white

is recognized

characteristic either

of rutile,

the exsolving

is associated

ellipsoidal Note

is suggestive

optically

of exsolution,

with

the reaction

2FeTi0

3

+

of Hem + R. The resulting ss to red internal reflections which are

and by the distinction

Hem

Hem

that the Hem Because

ss and Ti-poor,

stage of Ilmss + R + Hemss illustrates

the preferred

the decomposition

dissolution

than that illustrated of Ilmss to anatase

of euhedral Ti02 crystals. A portion of the unaltered upper left hand corner of (h). 0.1 mm.

to the exsol-

from color differences

of Fe203 and the development

Hg-124

are whiter than ss the decomposed hematite

in contrast

which are titanohematites. Hem Apart ss also have a higher reflectivity. 0.13 mm.

This series

' and the ss in the ilmenit

intergrowth

by yellow

or the exsolved ss 3 with rutile, it is Fe +-rich

A more advanced

are Hem

0.09 mm.

ution-associated the former

bodies

that R alone is absent

11m

Ilmenite of Hem ss rim, show decomposition to an intimate

assemblage

to Hem (c) ss which are

; the mantle is ilmenite with fine exsolution ss within the core as well as ilmenite in the same outer

bodies

(h-j)

0.15 mm.

0.15 mm.

is rutile.

1/202 = Fe20 + 2Ti02 is more likely. 3

(g)

The lamellae

Deformed Mt lamellae in picroilmenite which show partial oxidation ss and dissolution of these lamellae as shown in (d). The darker mantles

lighter

(f)

Reactions

are Ilmss' and the oriented lamellae along {0001} planE to complete decomposition to titanomaghemite. Note the

show partial

free of lamellae (e)

ss

in (h).

(Ti02) + Hem

of a porous

ilmenite

750 µm.

ss network

is shown in the

Figure Hg-3l

Hg-12s

UlvDspinel-Magnetite

ss

(a)

A parquet-textured micrograph

inter growth

but brown

of exsolved

in reflected

which must have been close to USPsOMtsO. (b)

Blitz-textured

The black results

although

rods are pleonaste

from a higher

Blocky

and incipient

Mtss

cally unresolvable

within

(Ramdohr,

areas.

of ex-

anisotropi( oxidation

These

ss

zone

lamellae

grains

the pleonast assemblage.

of Ilmss whereas Mtss

weak

and although

optical

anisotropy

USPss.

These

areas are dark

Ilmss are once again inferred. to describe marginal

to protoilmenite,

these opti-

texture grain

which is

(g) also has

0.15 mm. this grain into:

(left); and a highly

and minor Uspss exsolution.

0.15 rom.

Hg-126

thE

in a cloth-like

0.15 mm.

show a distinct

In addition

+

gradients

portion,

original

anisotropy;

from Mtss; ss 0.15 mm.

of Uspss

1963) has been employed

lath of Ilmss divides

and protoilmenite-rich

abundant

the Uspss-rich

optical

USPss

steep oxidation

finely dispersed

very distinct

lath of rIm

A thick sandwich

lamellae

The planes

is weakly

than the Usp-Mt

equal proportions

of extremely

also in (a) and in (e).

a sandwich

0.15 mm.

as discrete

to pleonaste.

Ilmss are hence inferred.

(white) with

The term protoilmenite

apparent

almost

cannot be observed

gray and display

(h)

solvi intersection

in zones adjacent

This is indicative

is apparent (f-g)

and the host is exsolved

temperature

are absent

area to the left contains

ilmenite

Uspss'

The assemblage

these cannot be identified

The area to the right of this grain contains

texture.

composition

0.15 mm.

Note that Uspss (e)

(gray in the phot,

grain of pyrrhotite.

to {100} spinel planes.

and Ilmss are inferred,

(d)

and USPss

from lit-par-lit Uspss to lamellar

series

are parallel

constituents.

(white)

0.15 rom.

Uspss in Mtss with an attached

(c) A transitional solution

Mtss

light oil iromersion) from an initial

oxidized

a relatively

unoxidized

zone with

Ilmss trellis

Figure Hg-32

Hg-127

oxidation

of the USPss

>600° and

The large adja

0.15 mm.

arrays of Al-rich

Coarse symplectic occurred

from Silicates

0.15 mm.

of amphibole.

O.ls rom.

crystal of olivine.

Oxidation

of associated

Pb

450 µm. ss

(f)

The outer margin mantle

of this highly oxidized

of Hem (white)

+

magnesioferrite

to this mantle is predominantly at high magnifications, that shown in grain (g-h)

These olivine having undergone

crystals

notations. phases

can be resolved which

are typical of those which are generally

The identifiable

blage parallels

magnetite

Because

is similar to

white phase is goethite

maghemitization

of titanomagnetite;

0.15 mm.

Hg-138

described

the name has only descriptive and the associated

in grain g) are smectite-related

as shown in Figure Hg-3sc.

zone adjacE

as

iddingsite is not mono-mineralic,

from the photomicrographs,

(note cleavages

(T > 600°C) has a diffusi

0.15 mm.

iddingsitization.

is clearly apparent

crystal

The highly reflective

Hem; the core of the olivine is dark and diffuse

symplectic

(e).

olivine (gray).

constituents.

ilmenite

as con-

darker This assem-

remains unaffected

Figure

Hg-36

Hg-139

plate-like

in form as illustrated

important Young,

constituent

in Figure Hg-36a.

in magnetic

studies,

1969; Evans and McElhinny,

of remanence crystals.

resides

common in pyroxenes McLelland,

metamorphism

a probable

by ga.rnet, develops

and Hg-36d

to magnetite;

during

temperatures.

and the assemblage,

as a corona around olivine

illustrate

the partial

(1971)

cooling

is

The products

of

which may also be

or at the grain junctions

decomposition

are typically

(Fig. Hg-36e-f).

form of silicate

results

of biotite

and of

very close to stoichiometric

formation

but is more clearly

may develop

between

Haggerty

and Baker,

magnetic

stability

assemblages

whereas

1967).

and remanence

cooling

weathering

pro-

oxides which show that the

by rutile + hematite,

to titanomaghemite

For all of the above cases,

which

for these olivine byproducts

iddingsite develops more typically

of titanomagnetite

which

(Fig. Hg-36g-h).

stages of deuteric

of supergene

of the discrete

suite is accompanied

is iddingsite

product

saturated

of olivine,

+ magnesioferrite

with smectite

high and low temperatures

in the oxidation

inversion

associated

to result as a consequence

magnetite-hematite-magnesioferrite hematite,

is the oxidation

or in hematite

the typical breakdown

intimately

during the volatile

demonstrated

The distinction

is also reflected

of goethite

decomposition

+ hematite

in magnetite

At lower temperatures

has a major constituent

the oxidation

increase

Fe 0 , 3 4 of 1-2 wt. % (MgO + Al203 + TiOZ) may also be present. is also very coromon and this leads to the formation of goethite.

The most widespread

+

at subsolidus

intergrowth

these magnetites

of these oxides

pseudobrookite

suggest that a pressure

concentrations

at high temperatures

cesses.

the result of

and olivine.

Figures Hg-36c

Iddingsite

are typically

have been made, for example by Griffin

the reaction

in a symplectic

plagioclase

but minor element

arguments

(Whitney and of olivine + plagio-

the reaction

These latter reactions

(1973), which

to trigger

result

accompanied

convincing

and Heier

mechanism

the reaction

Hydration

than in the larger discrete

but appear to be rare in plagioclase

+ spinel.

pyroxene

and

component

in the form of green or brown spinels are also relatively

(Fig. Hg-36b)

although

and by Griffin

amphibole

arrays rather

that the major stable

1973) in cases other than those which involve

clase = aluminous

between

oxides

oxides are an

(e.g., Hargraves

investigators

1969) have demonstrated

in these microscopic

More aluminous

These finely textured

and several

or by

in parallel

with

(Baker and Haggerty,

the earlier

comments

1967;

related

to

apply here also.

PRIMARY

OXIDE DISTRIBUTIONS

Introduction Paragenetic

sequences,

igneous rocks depend

compositions

and model abundances

of primary

for the most part on the initial bulk chemistry

the depth of emplacement,

and on the prevailing

oxygen fugacity

opaque oxides in

of the host rock, on

of the crystallizing

magma,

As a general principle because FeO, Ti0 and Cr 0 contents increase with decreasing 2 2 3 Si0 , basic rocks tend to contain larger concentrations of oxides than either intermediate 2 suites or acid end members. To a first approximation this distribution is controlled largely by the increases positions. determine

Titanium

in FeO contents

variations,

both the distribution

system FeO-Fe 0 -Ti0 . 2 3 2

Members

which result with increasingly

more mafic com-

on the other hand, in the range from acid to basic suites and composition

of mineral

solid solutions

of each of the solid solution

Hg-140

within

series Fe Ti0 -Fe 0 2 4 3 4

the

in the ratio of F~2+:Fe3+.

Therefore,

initial

and on f02'

titanium

hematite-rich

abundances

11m

oxide distributions These coupled

and compositions

parameters

depend on

lead to Mt-rich

and ss rocks, and to Usp-rich and 11m-rich in ss 2+ 3+ ss both Fe :Fe and Fe:Ti ratios are dependent on

in acid and intermediate

ss basic and ultrabasic

suites.

Because

temperature initial

and f02' for which coequilibrated oxide pairs yield unique solutions, the temperatures of crystallization and the cooling paths followed within the sub-

solidus

are the limiting

titanomagnetites initially

factors

lower temperature

tions + Ilm

This distribution

for compositional implies,

e.g., acid extrusives,

origins,

of closely

series are restricted

comparable

compositions.

variations

therefore,

Members

between

that suites of

result in Mt-rich

' but by the same token suites which reequilibrate

ss result in products

products,

chiefly responsible

and ilmenites.

solid solu-

over long periods

of time

of the Ti-enriched

FPb-Pb ss oxidation

to more basic rock types and are more abundant

as secon~ary

of Usp-Mt

(rutile,

and Ilm-Hem ' than as primary precipitates. The Ti0 polymorphs ss ss 2 brookite) are also commonly of secondary origin but minor primary con-

anatase,

centrations

are occasionally

present

in granites,

syenites

dances vary from low to very low in acid and intermediate character,

and peak in the ultrabasic.

almost exclusively MgO in chromite

to the basalt

(FeCr20 ) 4

Chrome-bearing

suite, whereas

and diorites.

members

more complex

are coromon and abundant

Chromium

rocks, increase of Mt-Uspss

are restricted

solid solutions

in picrites,

abun-

with more mafic

of A1 0 and 2 3 dunites and

peridotites,

in kimberlites. The major

controls

are the effects

on the distribution

of crystal

settling

that may exist in f02 levels batholiths,

sition,

extrusives,

at hypabyssal

levels

tling, the oxides assume radically These textures

remain

textural

capacities

placement.

retention

Volatile

also applies

With respect

is

of crystal

of the mode of emplacement

the problem

compoto suites

with rare exceptions,

relationships

as

fractionation

set-

but

with the silicates.

to inequigranular to the effects

in dikes and

of oxidation

is one of open and closed systems

of the host rock to volatile

loss during magmatic

em-

at depth in the closed system model will lead to higher 3 and greater proportions of Fe +, and with the higher probability of an approach

to equilibrium silicates.

for the partitioning

In a generalized

and of partial

the plutonic

of iron species

sense the extrusive

disequilibrium

ditions may vary between

among oxides

and of titanium

situation

and silicates.

these two extremes

although

between

oxides and

is one of rapid volatile In hypabyssal

it is probably

regions,

closer

loss

con-

to that of

environment.

The final and perhaps and compositions

oxygen fugacity f02 is a major elemental

Crystal

In the absence

in intrusives,

with depth,

and on the retention

abundance

and paragenetic

in extrusives.

control on oxide distribution

differences

which have evolved

distribution

equivalents,

regardless

may vary from equigranular

sills, and to interstitial

oxidation

component

of depth dependence

less common in rocks of intermediate

thick basic lava flows.

a minor

different

tephra.

This generalized

and for volcanic

absent in all but exceptionally

chemistry

or explosive

in basic intrusives,

and rare in acid intrusives.

intruded

as a function

and the likely but controversial

among rocks of equivalent

minor intrusions,

of oxides is widespread

of oxides

processes

the most fundamental of opaque oxides

and the f02 path which controlling

partitioning

factor

between

parameter

in igneous

is followed with

in elemental

the oxides

in controlling

cooling.

partitioning

and silicates. Hg-141

the distribution,

rocks is the level of initial At magmatic

among the oxides, For high values

temperatures and in

of f0 , oxides 2

f02 the available

iron is competitively

of titanium

partitioned

between

in the Bowen trend and the latter

The former results

and for coexisting

oxide solid solutions

oxides

these crystallizing

in the Fenner trend.

and silicates,

titanium

phases.

In the case

will preferentially

enter

of f02, and hence the precipitation of Fe Ti0 -, 2 4 FeTi0 -, and FeTi20s-rich solid solutions are favored. With progressively increasing 3 f0 , which is the generalized progression from basic to acid suites, the ratio of Fe3+:Fe 2 also increases, and this results in the stabilization of Mtss and of Hem-rich Ilmss The is also a strong undersaturated extent

at low values

interdependence

rocks.

that members

or perovskite

activities

values

of a

in defining of internal

retention

versus

external

and unknown

the nature

parameters,

buffering,

Either

(CaTiSiO ) s at relatively high

sphene

or whether

but some insight

and compositions

as a function

of depth is clearly

also in evaluating

that is, whether

crystallization

of the environment.

lower

of crucial

sig-

the relative

the environment

contri-

controls

itself has the overriding

The problem

is to be gained

effect

is complex with many variable

in the context

or rock type, as discussed

text of TOC and f02 of coequilibrated

in highl:

in Uspss and in Ilmss to the

absent.

as a function

f02 levels and is important

the trend or crystallization, in controlling

butions

and f02 which dominates

is depleted

2 are commonly

of the Ilmss series

and f02. Si02 The degree of volatile

butions

silica activity

Ti0

(CaTi0 ) may precipitate, with the former developing 3 and high oxygen fugacities, and the latter at correspondingly

silica

nificance

between

In these suites

oxide pairs as discussed

of oxide distri-

below,

and in the con-

in a later section

entitle,

T and f02 Variations in Igneous Rocks. Chromian

spinel distributions

Spinel,

chromian-spinel

to mafic and ultramafic and depleted

in Fe + Ti.

of early formed

crystals

in Fe + Ti enrichment crystallization cipitation cretely

and related

suites,

of plagioclase.

crystallizing

Complex

zoning

the partitioning Late-stage

spinels

reaction

(Mg and AI), but it is important of olivine

or of pyroxene

Al is accounted (Figs. Hg-2s

result

and pyroxene

for by the pre-

and Hg-26)

and dis-

in the more refractory

that before the onset of crystalproportion

of the available

Cr in

is locked up in early chromite.

To illustrate

the complexity

which are those from kimberlites,

of spinel

for the more general but varied

aspects

to note initially

of the multicomponent

trends exhibited

are the general

later enrichment in Al--the kimberlite 2 in Fe +--magmatic ore deposit xenolith

the most extreme

in Figure Hg-37,

in other petrologic

variations

These trends are: trend;

in Cr--the

and peridotite

(1) early enrichment

(2) early enrichment

individual

suites differ from one complex

trend.

to another, Hg-142

Starting

suites

tables.

The

in trends along the bases of Cr and

of Mg and later en-

trend; and (3) early enrichment

enrichment

examples,

and these provide

from data given in the accompanying

spinel prisms.

richment

zonal trends,

are shown in detail

which are suromarized in Figure Hg-38 important

to emphasize

an interaction

These reactions

The onset of olivine

depleted

exclusively

in Cr, Al and Mg

and this reflects decreases.

mantles

that a substantial

almost

are enriched

of Mg, whereas

are thus most commonly

lization

a contrast

is widespread,

as temperature increases in Fe3+.

elements

the magma

are restricted

with the liquid

with specific

dominate

spinel species

and as early precipitates

of Al and later

and terminal

as do the subtleties

points within in trend

«

•

Hg-143

.... .,., ....

..c::

0-

.-< OJ soooe is considered

pumice

above.

and plot above the NNO buffer

associated

suite,

+ Opx +

are similar to the c50ling paths defined by the slope of

those reported by earmichael biotite

pumice

(1967a,b) which

in T and fO

here

Table) •

phenocrysts

on the FMQ buffer

with data reported by earmichael

rock types, although

inasmuch

falls precisely

below that

are lower temperature

fH 0 at 73soe and 74Soe = 1100 and 1300 bars respectively The~ T-f

here

For the most

(see appropriate

when compared with the explosive

The rhyolites

reported

which

is separated

of the two suites shows that the rhyolites

where T = 860-890oe.

or straddle

and is calculated

(3) Data for the pitchstone

is close to, slightly

curve.

also includes

(T = 73S-7800e)

are iron-

the Opx group fall above the FMQ curve; and above this is the

is for one sample of a suite of rocks from the Thingmuli includes

group fallon

magma mixing

to have had Postvariations as a result

3

o

3

u

01';""'1°"' CO~COON co co '" 0'1

"'10 °1"' °1"' "'1°1"'SCO1°

N"'

O'\I""--CO

0'\

xl

13

....... ""COUi\()O'\N"" ....... r--CO

CO

CO

x

x

-c c

!

~ U

H

·I·I~I~ ~I~

.................. >, ........... "t:I"'d'"d..c"O ...... 0 III III III 1-1 g'} e-, ...c..o..oO.D,J:: 0,"

...... ...... ...... c.. ......

Hg-IBO

Pumice, Ash, Ignimbrites NOTES: (3)

(1)

This pumice

curve

Wet chemical

analysis.

falls on earmichael's

(see note 4, Acid Extrusive

includes and T

a suite of rhyolites. 860-8900e.

=

(5)

Table).

The two T-f

Oz tephra which have identical

differences

in 11m

(Mt84 vs

Electron microprobe

(4)

The rhyolites

rhyodacitic

Mtss

(2)

This pumice

mineralogies,

mapping

of tephra based

from a bi-lobate

but based on the

temperature.

(8)

The entries

Tuff which

and the Tira Canyon member; Rainer Mesa member

for Lipman

comprises

(7)

Identification

tuff units

Spring Member,

and (b) Timber Mt. Tuff which Tanks member.

properties

(a)

the Yucca Mt. member,

comprises

the

For four of the five members

+ qtz latite units show that the rhyolitic tuffs (i.e. higher

have crystallization

temperatures

which

of

T = blocking

are for two major

the Topopals

and the Ammonia

and coexisting

on the thermomagnetic

or "ferromagnetic-tephrochronology".

rhyolitic

and biotite-bearing

values are for samples

Fe-Ti oxides,

Paintbrush

study also

are amphibole

ss contents (Ilm73•S Hem26.S vs IlmSl Hem4l) Mt82) two distinct events are postulated. (6)

and stratigraphic

analysis.

Opx curve and is close to the NNO buffer

SiOz)

are lower than those of the associated

qtz latites,

and the relationship between inferred TOe and Si0 is very nearly 2 (low T and high Si02, 78 wt%; high T and low Si02' 66%). MnO (1-8 wt% in Ilmss; 1-4 wt% in Mtss) and A1203 (0.2-1.5 wt% in Mtss) decrease and increase linear

respectively

as T increases

(Buddington

and Lindsley,

1964).

Their respective

relationships

with Si02 therefore are that: A1 0 in Mtss increases as Si0 2 3 2 and MnO in 11m increases as Si0 increases. The range of T-f 2 ss 02 for the pumice suite falls along a curve which is closely parallel to the

decreases;

synthetic buffer curves and is between the Ni-NiO curve and the MnO-Mn 0 curve. 3 4 Values for Mtss with oxidation exsolution Ilmss yield T = 6S00e, and 7 12 8 10-16• atms; and T = 7800, and 10- • atms. In a number of rhyolitic tuffs the uppermost assemblage

portions

of the unit contain Mtss + sphene rather than the

Mtss + Ilmss.

This change in assemhlage[(which

is accompanied

also

(3eaTiSiOS + Fe304 = 3FeTi0 + 3eaSi03 + 1/2 02)]is indicative of 3 more highly oxidizing conditions and suggests that the upper units of each

byepx:

cogenetic

ash-flow

sequence

of the initial eruptions.

represents

500 to 1200 bars respectively. upper rhyolitic

a more highly

Inferred water

pressures

oxidized for T

The oxide data suggest

=

magma than those

62S-72Soe

range from

that the differentiated

(high Si0 and low modal phenocrysts) of the magma 2 SOOe) were very nearly saturated in H 0; the lower 2 portions of the chamber are represented by the qtz latite (low Si0 , high modal 2 phenocrysts) suite (T = 9000e) and these crystallized at low values of PH chamber

(T

=

parts

7000

±

2°· (9)

This study is an important

as it is directed

extension

of the above

(Lipman,

1971) inasmuch

an oxygen isotope comparison of TOe for the rhyolite 18 and qtz latite tuff suites. 00 values (per mil) in Mt range between ss 2.0-3.4 (qtz latites) and 0.3-2.S (rhyolites); feldspar + Mtss yield the respective

towards

ranges in temperature

quoted

in the Table.

Hg-18l

The upper limits of

.

co

,

": ~I ,,:1,,: ": ..;t ,...., I

0 .--I I

-.:tLl"'l .--1..-1 I I

"'.

"'.

M ,...., I

3

3

''',,,,,"''If''I ..:tll"lO'\O'>

.- 0

"""

I

Q'\

0'1

.-
50 wt% Cr203, < 20 wt% A1203) are randomly distributed along the strike of the complex and are restricted to dunites. Pods of Al-rich chromite (- 20 wt% A1203) are interspersed and are characteristically present in association with other mafic rocks (harzburgites) in the peridotite. The Al-rich pods are considered to be younger than the Cr-rich pods but the disposition of these pods in the protocomplex is conjectural. The complex is modelled on the early formation of Cr-rich chromite in dunite and later accumulation of Al-rich chromite in a pyroxenitic-mush. Compositions of Selected Coolac Chromites 1 2 3 4 5 6 7 8 9 10 11 12 33.3 36.3 42.5 45.6 49.5 50.8 53.2 56.5 58.2 60.0 62.6 58.0 35.3 30.7 27.4 22.1 19.8 17.4 17.0 12.2 11 .5 8.5 5.6 5.4 5.2 5.8 1.9 4.9 3.2 9.5 2.5 5.6 5.3 2.9 5.9 7.3 8.6 8.8 12.2 12.1 15.8 6.9 13.2 14.0 11.0 19.5 12.6 23.2 17.6 18.4 16.0 15.3 11.7 15.4 14.1 11.7 14.0 9.1 13.3 6.1

Coolac Dlstrlct, N.S.W. Ai:iS'tr alia

Golding and Johnson (1971 )

Hutchison (1972)

Chromite layers and pods in dunite and serpentinite. Review of earlier data and new analyses for the region show that Cr203 varies between 31.4-55.76 wt%; A1203 between 8.9-27.4 wt%; MgO between 7.99-19.05 wt%; Fe203 between 0.22-39.28 wt% and Ti02 between 0.03-0.75 wt%. All ultramafic bodies in the region contain significant concentrations of Cr (2770 ppm) and Ni (1530 ppm) and yet the chromite ore bodies have a restricted distribution. A mantle origin is proposed for the ore bodies with subsequent emplacement as tabular masses into the crust.

Darvel Bay, North Borneo

References

Chromite ore bodies are present in dunites and harzburgites and display a variety of Rodgers (1973) textural forms from massive to disseminated and orbicular, which are in part consistent with a magmatic cumulative origin. The harzburgites contain < 1% of a chromianspinel (28-45% Cr203) in association with olivine (F087-92) and Opx (En89-92); dunites contain 1-3% picrochromite (39-50% Cr203) in association with F067-93; the chromitites also contain picrochromites (42-59% Cr203), and serpentinized ollvine (F092-96). Temperatures of crystallization are considered to have been - 12000C, which contrasts with picrochromite-bronzite symplectites which are considered to have formed at - 7900C. Although classically an alpine type association, a cumulate origin is favored.

CHROMIAN SPINELS

Massif du Sud, Southern New Caledonia

Local ity

MAFIC AND ULTRAMAFIC ASSOCIATIONS

MAGMATIC ORE DEPOSITS

Table Hg-19 (7)

"

c-

N V>

I

OQ

CHROMIAN SPINELS

Bushveld Disseminated Chr in pyroxenite Chr bands in pyroxenite

Great Dike Disseminated Chr in Harz and Pic Chr - seam #2 Chr - seam #1

41.8 - 46.5 35.6 - 47.3

Cr203 48.0 - 53.7 55.1 - 57.3 51.0 - 51.68

0.84 - 1.62 0.87 - 1.59

Cr/Fe 1.67 - 2.32 2.37 - 2.73 1.93 - 2.06

The great dike consists of 4 ultrabasic lopolithic complexes. Each complex is synclinal and is divided into the following units: Unit 1 (top) is gabbroic; Unit 2 is pyroxenite, olivine in basal part, picrit~harzburgite, chromite seam #1, harzburgite, chromite seam #2; Unit 3 (bottom) is pyroxenite. Disseminated chromites in horizons adjacent to, above and below seams #1 and #2 are as follows in ascending order: Lower Upper Harz Harz Below Above Oxide Harz Seam #2 Harz Seam #1 Picrite Cr203 50.30 *55.09 - 57.25 50.98 53.7ll52.80 *50.40 - 48.00 A1203 15.45 11.80 11.05 13.94 12.70 14.35 14.96 15.58 Fe203 2.74 2.16 1.74 3.30 4.29 1.52 4.00 5.18 FeO 20.88 18.54 18.00 22.22 20.24 18.61 21.83 20.59 MgO 9.26 10.24 9.87 7.25 7.35 10.40 7.00 7.10 Ti02 0.78 0.80 0.54 1.07 1.05 0.47 0.88 1.36 MnO 0.35 0.31 0.27 0.35 0.44 0.38 0.36 0.34 CaO 0.08 0.12 0.21 0.10 0.16 0.12 0.13 0.09 Si02 1.40 1.78 1.38 1.35 1.31 1.74 1.33 1.47 101.24 100.84 100.31 100.56 101.24 100. 39 100.89 99.71 *Choice of analysis based on min and max values for Cr203. The major variations between the disseminated and the massive chromite bands are: massive chromite contains higher Cr203, lower A1203, and lower FeO, but comparable MgO and Ti02 contents. A comparison of the great dike chromites with those in the Bushveld and Stillwater complexes are as follows:

Hartley Complex, Great Dike, Rhodesia

Locality

MAFIC AND ULTRAMAFIC ASSOCIATIONS

MAGMATIC ORE DEPOSITS

Table Hg-19 (8)

Bichan (1969)

References

"

~

N

I

OQ

-

1.38 99.81

-

0.23

0.53 99.72

0.27

-

-

-

-

7.54 12.53 27.37 13.81 33.25 5.08 -

-

3.92 16.55 32.13 11.15 31.61 4.49

-

-

-

-

3.21 20.49 31.07 7.93 31.69 4.02

-

9 0.50 1.90 14.62 37.32 16.37 21.19 8.00

-

1.28 12.18 41.68 12.65 27.19 5.20

-

10

-

2.07 14.47 38.59 12.77 25.37 7.12

-

T 11

0.42 0.14 1.50 0.81 0.37 0.34 99.90 99.22 100.65 100.55 100.73 BUSHVELD -

-

7.94 13.30 27.42 10.07 36.86 2.81

-

Atypical in Ti+Cr+Al 5 6 7 8

3.90 10.20 1.61 1.45 99.67 101.69 101.19 101.30

-

Ty~i ca 1 + Atypical in V 1 2 3 4 0.34 0.88 15.38 11.30 7.10 10.37 3.55 2.99 1.79 6.20 0.08 0.18 2.04 5.28 39.11 45.66 47.04 27.35 39.15 36.64 37.51 41.75 1.31 0.54 0.55 0.29

0.20 99.33

-

-

0.59 14.77 47.98 7.10 18.87 10.42

ical 12

0.19 99.72

-

-

0.43 13.27 49.43 6.82 20.55 9.03

13

-

0.47 98.02

-

1.03 15.87 42.81 8.22 19.93 9.68

14

Av Ty~ical -1-715 16 0.34 0.23 1.60 0.91 0.6 13.0 14.4 16.6 49.2 36.6 46.2 3.1 16.0 25.3 24.0 25.0 6.5 7.2 11 .1 0.3 0.01 0.31 0.19 0.3 0.09 0.15 1.10 0.17 100.9 gg:g- 99.8 STILLWATER AVERAGE STRATIFORM

Bichan (1969)cont.

References to analyses: (1-2) Molyneux (1972). Analyses 1 and 2 are from the eastern lobe of the Bushveld and are at 3084' {anorthosite} and 3610' (seam #11) respectively above the base of the Merensky Reef in the Upper Zone; (3-8) Cameron and Glover (1973). Samples are from "replacement" pegmatites in the Critical Zone and bridge the gap between Ti-poor chromites at the base of the Bushveld and titanomagnetites of the upper part. (9) Van Zyl (1969). Merensky Reef. (10-14) Cameron (1975), Critical Zone. Analysis #10 in plag.; #11 in bronzite; #12 intersertial chromite; #13 in olivine; and #14 is massive chromite. (15-16) Thayer (1969). Analysis #15 = G-zone; and #16 = B-zone in layered chromites. (17) Dickey and Yoder (1972). This analysis is the average stratiform chromite composition based on 3 complexes and 45 analyses.

Oxide Si02 Ti02 A1203 Cr203 Fe203 FeO MgO CaO MnO NiO V203 Total

Re~resentative and Exotic Spinel Compositions in the Bushveld Complex

Origin and conclusion suggests that the chromite bands in Unit 2 of the Great Dike resulted from two discrete magmatic events and that convective overturning was minimal or absent.

1 .23 - 1.57 1.69 - 1.99

Table Hg-19 (9)

Stillwater Disseminated Chr in dunite or Harz 40.5 - 47.82 Chromite bands in Harz 46.02 - 50.98

"

N V> 00

I

OQ

99.05

-

5.7 41. 2